sync compositor: Signal async frame on IO thread

android_webview/browser/child_frame.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ANDROID_WEBVIEW_BROWSER_CHILD_FRAME_H_
 #define ANDROID_WEBVIEW_BROWSER_CHILD_FRAME_H_
 
 #include <memory>
 
 #include "android_webview/browser/compositor_id.h"
@@ skipping 12 matching lines @@
  public:
   ChildFrame(uint32_t compositor_frame_sink_id,
              std::unique_ptr<cc::CompositorFrame> frame,
              const CompositorID& compositor_id,
              bool viewport_rect_for_tile_priority_empty,
              const gfx::Transform& transform_for_tile_priority,
              bool offscreen_pre_raster,
              bool is_layer);
   ~ChildFrame();
 
-  const uint32_t compositor_frame_sink_id;
+  // These two fields are not const to make async path easier.
+  uint32_t compositor_frame_sink_id;
   std::unique_ptr<cc::CompositorFrame> frame;
   // The id of the compositor this |frame| comes from.
   const CompositorID compositor_id;
   const bool viewport_rect_for_tile_priority_empty;
   const gfx::Transform transform_for_tile_priority;
   const bool offscreen_pre_raster;
   const bool is_layer;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ChildFrame);
 };
 
 }  // namespace webview
 
 #endif  // ANDROID_WEBVIEW_BROWSER_CHILD_FRAME_H_