cc/blimp: Add synchronization for scroll/scale state.

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 "client_state_update.proto";
 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
   // Sent from the client to the engine when a frame update was processed
   // on the client.
   optional bool frame_ack = 4;
 
+  // Client -> Engine
+  // Reports any changes made to the main thread state on the compositor thread
+  // on the client.
+  optional ClientStateUpdate client_state_update = 5;
+
+  // Engine -> Client
+  // Acknowledges the application of the client state update to the associated
+  // state on the engine.
+  // If the application of the update resulted in any additional changes to the
+  // state on the engine, the ack will always be bundled with the resulting
+  // frame update.
+  optional bool client_state_update_ack = 6;
+
   // One of these is set based on where the message is going to.
   optional CompositorMessageToMain to_main = 1;
   optional CompositorMessageToImpl to_impl = 2;
 }