Content "gn check" work.

content/renderer/BUILD.gn

Index: content/renderer/BUILD.gn
diff --git a/content/renderer/BUILD.gn b/content/renderer/BUILD.gn
index bf4554614035c6678d0b90d351b418bba94da199..fa80b4045fce73b359feadf14c7cf690db603a59 100644
--- a/content/renderer/BUILD.gn
+++ b/content/renderer/BUILD.gn
@@ -10,7 +10,10 @@ import("//media/media_options.gni")
 source_set("renderer") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
-  visibility = [ "//content/public/renderer:renderer_sources" ]
+  visibility = [
+    ":for_content_tests",
+    "//content/public/renderer:renderer_sources",
+  ]
 
   sources = rebase_path(content_renderer_gypi_values.private_renderer_sources,
                         ".",
@@ -247,3 +250,13 @@ source_set("renderer") {
     deps += [ "//ui/ozone" ]
   }
 }
+
+# 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 = [
+      ":renderer",
+    ]
+  }
+}