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",