Add closure_compile build flag.

This CL adds a closure_compile build flag which is set to compile on
the android, linux and chromeos trybots. This flag causes the js_type_check
target to no-op by generating an empty group() target.

Bug: 820473
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I77e2fdbb9bb7881cf0cb335bad0978c8532ea539
Reviewed-on: https://chromium-review.googlesource.com/958308
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546398}

diff --git a/BUILD.gn b/BUILD.gn
index d7058700..1fdb3ab 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -21,6 +21,7 @@
 import("//third_party/openh264/openh264_args.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 import("//ui/base/ui_features.gni")
+import("//ui/webui/webui_features.gni")
 import("//ui/ozone/ozone.gni")
 import("//v8/gni/v8.gni")
 import("//v8/snapshot_toolchain.gni")
@@ -89,6 +90,10 @@
     ]
   }
 
+  if (closure_compile) {
+    deps += [ ":webui_closure_compile" ]
+  }
+
   if (!is_ios && !is_android && !is_chromecast && !is_fuchsia) {
     deps += [
       "//chrome",
@@ -1153,7 +1158,7 @@
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
-if (!is_win) {
+if (closure_compile) {
   group("webui_closure_compile") {
     data_deps = [
       "chrome/browser/resources/engagement:closure_compile",

diff --git a/chrome/browser/resources/engagement/BUILD.gn b/chrome/browser/resources/engagement/BUILD.gn
index 93c8c3a1..84ee3210 100644
--- a/chrome/browser/resources/engagement/BUILD.gn
+++ b/chrome/browser/resources/engagement/BUILD.gn

@@ -4,13 +4,13 @@
 
 import("//third_party/closure_compiler/compile_js.gni")
 
-group("closure_compile") {
+js_type_check("closure_compile") {
   deps = [
     ":site_engagement",
   ]
 }
 
-js_binary("site_engagement") {
+js_library("site_engagement") {
   deps = [
     "//ui/webui/resources/js:assert",
     "//ui/webui/resources/js:util",

diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index bfdee74e..2c6bb35 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni

@@ -14,6 +14,7 @@
 import("//ppapi/buildflags/buildflags.gni")
 import("//printing/features/features.gni")
 import("//ui/base/ui_features.gni")
+import("//ui/webui/webui_features.gni")
 
 assert(!is_ios, "Chromium/iOS shouldn't use anything in //chrome")
 

diff --git a/third_party/closure_compiler/compile_js.gni b/third_party/closure_compiler/compile_js.gni
index 83a2f1d3..c1955d8b 100644
--- a/third_party/closure_compiler/compile_js.gni
+++ b/third_party/closure_compiler/compile_js.gni

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//third_party/closure_compiler/closure_args.gni")
+import("//ui/webui/webui_features.gni")
 
 script_path = "//third_party/closure_compiler"
 compiler_path = "$script_path/compiler/compiler.jar"
@@ -219,8 +220,14 @@
 
 # Defines a target that compiles a group of js_library targets.
 template("js_type_check") {
-  js_binary(target_name) {
-    sources = []
-    forward_variables_from(invoker, [ "deps" ])
+  if (closure_compile) {
+    js_binary(target_name) {
+      sources = []
+      forward_variables_from(invoker, [ "deps" ])
+    }
+  } else {
+    not_needed(invoker, "*")
+    group(target_name) {
+    }
   }
 }

diff --git a/ui/base/ui_features.gni b/ui/base/ui_features.gni
index 1a8ec2b0..25f9980 100644
--- a/ui/base/ui_features.gni
+++ b/ui/base/ui_features.gni

@@ -20,11 +20,6 @@
   # Set to true to if mus (aka the UI service) is enabled. Use the features kMus
   # (or kMash in chrome code) to start in mus/mash.
   enable_mus = is_chromeos
-
-  # Optimize parts of Chrome's UI written with web technologies (HTML/CSS/JS)
-  # for runtime performance purposes. This does more work at compile time for
-  # speed benefits at runtime (so we skip in debug builds).
-  optimize_webui = !is_debug
 }
 
 enable_hidpi = is_mac || is_win || is_linux

diff --git a/ui/resources/BUILD.gn b/ui/resources/BUILD.gn
index 8286ba5..c10eb455 100644
--- a/ui/resources/BUILD.gn
+++ b/ui/resources/BUILD.gn

@@ -5,7 +5,7 @@
 import("//build/config/jumbo.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
-import("//ui/base/ui_features.gni")
+import("//ui/webui/webui_features.gni")
 
 group("resources") {
   public_deps = [

diff --git a/ui/webui/webui_features.gni b/ui/webui/webui_features.gni
new file mode 100644
index 0000000..c20d7c3
--- /dev/null
+++ b/ui/webui/webui_features.gni

@@ -0,0 +1,15 @@
+# Copyright 2018 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+declare_args() {
+  # Optimize parts of Chrome's UI written with web technologies (HTML/CSS/JS)
+  # for runtime performance purposes. This does more work at compile time for
+  # speed benefits at runtime (so we skip in debug builds).
+  optimize_webui = !is_debug
+
+  # Enable closure type-checking for Chrome's web technology-based UI. This
+  # enables the webui_closure_compile target which does a no-op without this
+  # flag enabled. Requires Java.
+  closure_compile = is_chromeos || is_linux || is_android
+}