Send FrameSwapMessageQueue's messages with a separate IPC

cc/output/compositor_frame_metadata.h

 // Copyright (c) 2012 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 CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_
 #define CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 74 matching lines @@
 
   // This is a value that allows the browser to associate compositor frames
   // with the content that they represent -- typically top-level page loads.
   // TODO(kenrb, fsamuel): This should eventually by SurfaceID, when they
   // become available in all renderer processes. See https://crbug.com/695579.
   uint32_t content_source_id = 0;
 
   // BeginFrameAck for the BeginFrame that this CompositorFrame answers.
   BeginFrameAck begin_frame_ack;
 
+  // Once the display compositor processes a frame containing a non-zero frame
+  // token, the token is sent to embedder of the frame. This is helpful when
+  // the embedder wants to do something after a particular frame is processed.
+  uint32_t frame_token = 0;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_