Content "gn check" work.

content/BUILD.gn

 # Copyright 2014 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.
 
 import("//tools/grit/grit_rule.gni")
 import("//build/config/nacl/config.gni")
 
 # Applied by targets internal to content.
 config("content_implementation") {
   defines = [ "CONTENT_IMPLEMENTATION" ]
@@ skipping 19 matching lines @@
 #     //content (shared library) ->
 #       //content/public/browser:browser_sources (source set) ->
 #         //content/browser (source set; this is the non-public browser target)
 #         //content/public/common:common_sources (source set)
 #
 # That the internal content dependencies must depend on the *_sources targets
 # to avoid dependency cycles, and external dependencies must depend on the
 # //content/public/browser and similar targets to avoid double-linking (these
 # targets make sure the dependency goes through the content shared library
 # when doing a component build).
+#
+# TESTS
+# -----
+# Tests are a challenge. The content tests need to access internals of
+# content/browser, for example, but the tests themselves are outside of the
+# content component (which is a shared library in the component build). To
+# prevent external-to-content targets from depending on private headers,
+# the non-public content/browser target is not a public dep of the content
+# component. But this means there is also no public path for the content
+# tests and no way to restrict that just to //content/test/* if we added one.
+#
+# As a result, we check deps for //content/test/* only in non-component builds
+# (which should verify the dependencies are correct for both component and
+# non-component cases equally). There are targets like
+# //content/browser:for_content_tests that allow content/test to depend on the
+# //content/browser target only in non-component builds (when there are no
+# linking problems) for when check is enabled.
 
 if (!is_nacl_nonsfi) {
   content_shared_components = [
     "//content/gpu:gpu_sources",
     "//content/public/browser:browser_sources",
     "//content/public/child:child_sources",
     "//content/public/gpu:gpu_sources",
     "//content/public/common:common_sources",
     "//content/public/plugin:plugin_sources",
     "//content/public/renderer:renderer_sources",
@@ skipping 42 matching lines @@
   use_qualified_include = true
   outputs = [
     "grit/content_resources.h",
     "content_resources.pak",
   ]
 }
 
 # This target exists to "hold" the content_export header so we can do proper
 # inclusion testing of it.
 source_set("export") {
+  # Must only be used inside of content.
   visibility = [ "//content/*" ]
   sources = [
     "common/content_export.h",
   ]
 }
 
 # In the GYP build, this file is listed in several targets. In GN just have
 # those targets depend on this one. This can be depended on for any
 # platform for simplicity, and is a no-op on non-Windows.
 source_set("sandbox_helper_win") {
   if (is_win) {
     sources = [
       "app/sandbox_helper_win.cc",
       "public/app/sandbox_helper_win.h",
     ]
 
     deps = [
       "//sandbox",
     ]
   }
 }