Content "gn check" work.

content/browser/BUILD.gn

Index: content/browser/BUILD.gn
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 9245309c446ae1210785fe541c47b04c666a6d82..fcd1572500ebb9a41adeb7d9374e67c0014ccf4a 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -11,7 +11,10 @@ import("//mojo/public/mojo_application_manifest.gni")
 source_set("browser") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
-  visibility = [ "//content/public/browser:browser_sources" ]
+  visibility = [
+    ":for_content_tests",
+    "//content/public/browser:browser_sources",
+  ]
 
   configs += [
     "//build/config:precompiled_headers",
@@ -526,6 +529,16 @@ source_set("browser") {
   }
 }
 
+# See comment at the top of //content/BUILD.gn for how this works.
+group("for_content_tests") {
+  visibility = [ "//content/test/*" ]
+  if (!is_component_build) {
+    public_deps = [
+      ":browser",
+    ]
+  }
+}
+
 mojo_application_manifest("chrome_manifest") {
   type = "exe"
   application_name = "chrome"