Add RenderFrameImplTest

Support for renderer-side browser tests for RenderFrameImpl.

BUG=

Review URL: https://codereview.chromium.org/1032623006

Cr-Commit-Position: refs/heads/master@{#323332}
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index c1bd46b..e00e9bf 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -556,6 +556,7 @@
   RenderFrameImpl(RenderViewImpl* render_view, int32 routing_id);
 
  private:
+  friend class RenderFrameImplTest;
   friend class RenderFrameObserver;
   friend class RendererAccessibilityTest;
   FRIEND_TEST_ALL_PREFIXES(ExternalPopupMenuDisplayNoneTest, SelectItem);