[cc] Pass on BeginFrameAcks from CompositorEBFS through RWHVAura, DFH.

cc/ipc/compositor_frame_metadata_struct_traits.h

 // Copyright 2016 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_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_
 #define CC_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_
 
 #include "cc/ipc/compositor_frame_metadata.mojom-shared.h"
 #include "cc/output/compositor_frame_metadata.h"
 
@@ skipping 98 matching lines @@
   static bool can_activate_before_dependencies(
       const cc::CompositorFrameMetadata& metadata) {
     return metadata.can_activate_before_dependencies;
   }
 
   static uint32_t content_source_id(
       const cc::CompositorFrameMetadata& metadata) {
     return metadata.content_source_id;
   }
 
+  static cc::BeginFrameAck begin_frame_ack(

[dcheng, 2017/03/17 05:54:02]

Return a const reference here--even though it's a POD, it's still not free to
copy

[Eric Seckler, 2017/03/17 11:06:44]

Done.

+      const cc::CompositorFrameMetadata& metadata) {
+    return metadata.begin_frame_ack;
+  }
+
   static bool Read(cc::mojom::CompositorFrameMetadataDataView data,
                    cc::CompositorFrameMetadata* out);
 };
 
 }  // namespace mojo
 
 #endif  // CC_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_