[ui/ws] Acknowledge BeginFrames in FrameGenerator + add tests.

services/ui/ws/frame_generator.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 SERVICES_UI_WS_FRAME_GENERATOR_H_
 #define SERVICES_UI_WS_FRAME_GENERATOR_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 68 matching lines @@
   // SetNeedsBeginFrame sets observing_begin_frames_ and add/remove
   // FrameGenerator as an observer to/from begin_frame_source_ accordingly.
   void SetNeedsBeginFrame(bool needs_begin_frame);
 
   FrameGeneratorDelegate* delegate_;
   ServerWindow* const root_window_;
   float device_scale_factor_ = 1.f;
 
   std::unique_ptr<cc::CompositorFrameSink> compositor_frame_sink_;
   cc::BeginFrameArgs last_begin_frame_args_;
+  cc::BeginFrameAck current_begin_frame_ack_;
   cc::BeginFrameSource* begin_frame_source_ = nullptr;
   bool observing_begin_frames_ = false;
 
   cc::SurfaceInfo window_manager_surface_info_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameGenerator);
 };
 
 }  // namespace ws
 
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_FRAME_GENERATOR_H_