Add //ipc to "gn check", work on base

This fixes the header issues in these directories and adds them to the list of targets to be checked.

Review URL: https://codereview.chromium.org/926593007

Cr-Commit-Position: refs/heads/master@{#316705}

diff --git a/.gn b/.gn
index 23013d4..1429a87 100644
--- a/.gn
+++ b/.gn

@@ -14,4 +14,11 @@
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
-check_targets = [ "//cc/*" ]
+check_targets = [
+  # Requires GN binary >= 314794 and to mark the "...__generate_enum" action in
+  # build/config/android/rules.gni "java_cpp_enum" template with
+  #   check_includes = false.
+  #"//base/*",
+  "//cc/*",
+  "//ipc/*",
+]