[WebView] Add a new flushVisualState API to AwContents.

content/public/browser/render_frame_host.h

Index: content/public/browser/render_frame_host.h
diff --git a/content/public/browser/render_frame_host.h b/content/public/browser/render_frame_host.h
index 3d1cf17bdcbf08b9f5de37ad360dc8402ce21137..e6a0069596d40713bd87bf082e08438f153892bf 100644
--- a/content/public/browser/render_frame_host.h
+++ b/content/public/browser/render_frame_host.h
@@ -93,6 +93,15 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // result.
   virtual void ActivateFindInPageResultForAccessibility(int request_id) = 0;
 
+  // Roundtrips through the renderer and compositor pipeline to ensure that any
+  // changes to the contents resulting from operations executed prior to this
+  // call are visible on screen. The call completes asynchronously by running
+  // the supplied |callback| with a value of true upon successful completion and
+  // false otherwise (when the frame is destroyed, detached, etc..).
+  typedef base::Callback<void(bool)> FlushVisualStateResultCallback;
+  virtual void FlushVisualState(
+      const FlushVisualStateResultCallback& callback) = 0;
+
   // Temporary until we get rid of RenderViewHost.
   virtual RenderViewHost* GetRenderViewHost() = 0;