Content "gn check" work.

content/app/BUILD.gn

Index: content/app/BUILD.gn
diff --git a/content/app/BUILD.gn b/content/app/BUILD.gn
index abe8a752254fe499c7fa39ff40e6e797edbacbde..149143920f6b5829fb5407e216c31e3fa381d9be 100644
--- a/content/app/BUILD.gn
+++ b/content/app/BUILD.gn
@@ -67,13 +67,26 @@ if (!is_multi_dll_chrome) {
 source_set("both") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
-  visibility = [ "//content/public/app:*" ]
+  visibility = [
+    ":both_for_content_tests",  # See top of //content/BUILD.gn for why.
+    "//content/public/app:*",
+  ]
 
   sources = content_app_sources
   configs += content_app_extra_configs
   deps = content_app_deps
 }
 
+# See comment at the top of //content/BUILD.gn for how this works.
+group("both_for_content_tests") {
+  visibility = [ "//content/test/*" ]
+  if (!is_component_build) {
+    public_deps = [
+      ":both",
+    ]
+  }
+}
+
 if (is_multi_dll_chrome) {
   # It doesn't make sense to do the browser/child dll split in component mode.
   assert(!is_component_build)