Content "gn check" work.

content/BUILD.gn

Index: content/BUILD.gn
diff --git a/content/BUILD.gn b/content/BUILD.gn
index 1d32ad24a201bd4f5ef13729755950ad2a915404..1ece4f97f491eb1ca10ff27a65a350fc71b3bcc1 100644
--- a/content/BUILD.gn
+++ b/content/BUILD.gn
@@ -37,6 +37,23 @@ config("content_implementation") {
 # //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 = [
@@ -99,6 +116,7 @@ grit("resources") {
 # 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",