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

blimp/engine/renderer/blimp_remote_compositor_bridge.h

Index: blimp/engine/renderer/blimp_remote_compositor_bridge.h
diff --git a/blimp/engine/renderer/blimp_remote_compositor_bridge.h b/blimp/engine/renderer/blimp_remote_compositor_bridge.h
index bfb36549900f25bc5826eec57913578bf90a55ae..d846d27bf6339859063a00ab77df2fea9c217a6e 100644
--- a/blimp/engine/renderer/blimp_remote_compositor_bridge.h
+++ b/blimp/engine/renderer/blimp_remote_compositor_bridge.h
@@ -33,6 +33,8 @@ class BlimpRemoteCompositorBridge
   void ProcessCompositorStateUpdate(std::unique_ptr<cc::CompositorProtoState>
                                         compositor_proto_state) override;
 
+  FrameScheduler* scheduler_for_testing() { return &scheduler_; }
+
  private:
   // cc::RemoteProtoChannel::ProtoReceiver implementation.
   void OnProtoReceived(
@@ -44,6 +46,8 @@ class BlimpRemoteCompositorBridge
   cc::RemoteProtoChannel* remote_proto_channel_;
   cc::RemoteCompositorBridgeClient* client_ = nullptr;
 
+  bool client_state_update_ack_pending_ = false;
+
   FrameScheduler scheduler_;
 
   DISALLOW_COPY_AND_ASSIGN(BlimpRemoteCompositorBridge);