Navigation transitions: Block first response until after transitions have run.

content/browser/frame_host/render_frame_host_delegate.h

Index: content/browser/frame_host/render_frame_host_delegate.h
diff --git a/content/browser/frame_host/render_frame_host_delegate.h b/content/browser/frame_host/render_frame_host_delegate.h
index b4db5d4c9c91cea306c7e13cd2a6590c962313fe..857ef1015e2b097368973fc64875f66be3450df8 100644
--- a/content/browser/frame_host/render_frame_host_delegate.h
+++ b/content/browser/frame_host/render_frame_host_delegate.h
@@ -55,6 +55,10 @@ class CONTENT_EXPORT RenderFrameHostDelegate {
   // The RenderFrameHost has been swapped out.
   virtual void SwappedOut(RenderFrameHost* render_frame_host) {}
 
+  // Notification that the navigation on the main frame is blocked waiting
+  // for transition to occur.
+  virtual void DidDeferAfterResponseStarted() {}
+
   // Notification that a worker process has crashed.
   virtual void WorkerCrashed(RenderFrameHost* render_frame_host) {}