content/blimp: Set up hooks for enabling LTHRemote in the renderer.

cc/proto/compositor_message.proto

 // 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 import "compositor_message_to_impl.proto";
 import "compositor_message_to_main.proto";
 import "layer_tree_host.proto";
 
 package cc.proto;
 
 // A top level message for serialized state used by the compositor remote
 // channels.
 // TODO(khushalsagar): Remove CompositorMessage and stop using
 // RemoteProtoChannel to plumb these to/from the browser. See crbug.com/648442
 message CompositorMessage {
   // Engine -> Client
   // Frame Update.
   optional LayerTreeHost layer_tree_host = 3;
 
+  // Client -> Engine
+  // Frame Ack.

[danakj, 2016/10/13 19:23:23]

The comment is literally the same text as the variable name, can you say what it
is instead of its name?

[Khushal, 2016/10/13 20:03:57]

Done.

+  optional bool frame_ack = 4;
+
   // One of these is set based on where the message is going to.
   optional CompositorMessageToMain to_main = 1;
   optional CompositorMessageToImpl to_impl = 2;
 }