PlzNavigate: add support in several navigation controller unit tests

content/test/test_render_frame_host.h

Index: content/test/test_render_frame_host.h
diff --git a/content/test/test_render_frame_host.h b/content/test/test_render_frame_host.h
index a828bd56a09fcbdf73ff57527d5967ae4bdae0b7..621bc6debebb57c896f0d0209dc394bf7701c5ac 100644
--- a/content/test/test_render_frame_host.h
+++ b/content/test/test_render_frame_host.h
@@ -95,6 +95,13 @@ class TestRenderFrameHost : public RenderFrameHostImpl,
     simulate_history_list_was_cleared_ = cleared;
   }
 
+  // Advances the RenderFrameHost (and through it the RenderFrameHostManager) to
+  // a state where a new navigation can be committed by a renderer. Currently,
+  // this simulates a BeforeUnload ACK from the renderer.
+  // PlzNavigate: this simulates a BeforeUnload ACK from the renderer, and the
+  // interaction with the IO thread up until the response is ready to commit.
+  void PrepareForCommit(const GURL& url);
+
  private:
   TestRenderFrameHostCreationObserver child_creation_observer_;