[Push] Use Manifest.gcm_sender_id instead of API sender_id if possible.

PushMessagingDispatcher is now trying to get the Manifest before
asking the registration to happen. If the gcm_sender_id is set, it will
be used. Otherwise, the flow is unchanged.

This is an considered option in order to no longer require an opaque
property bag to be passed when registering. It might or might not stay
as is.

BUG=414873

Review URL: https://codereview.chromium.org/609393002

Cr-Commit-Position: refs/heads/master@{#297403}
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index 76a373a..7dd2860 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -461,6 +461,8 @@
   void BindServiceRegistry(
       mojo::ScopedMessagePipeHandle service_provider_handle);
 
+  ManifestManager* manifest_manager();
+
  protected:
   RenderFrameImpl(RenderViewImpl* render_view, int32 routing_id);