Use swap-promise to improve first* paint times

content/renderer/gpu/render_widget_compositor.h

 // Copyright (c) 2013 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 CONTENT_RENDERER_GPU_RENDER_WIDGET_COMPOSITOR_H_
 #define CONTENT_RENDERER_GPU_RENDER_WIDGET_COMPOSITOR_H_
 
 #include <stdint.h>
 
 #include "base/callback.h"
@@ skipping 149 matching lines @@
   void UpdateEventRectsForSubframeIfNecessary() override;
   blink::WebEventListenerProperties EventListenerProperties(
       blink::WebEventListenerClass eventClass) const override;
   void SetHaveScrollEventHandlers(bool) override;
   bool HaveScrollEventHandlers() const override;
   int LayerTreeId() const override;
   void SetShowFPSCounter(bool show) override;
   void SetShowPaintRects(bool show) override;
   void SetShowDebugBorders(bool show) override;
   void SetShowScrollBottleneckRects(bool show) override;
+  void NotifySwapTime(base::Callback<void(bool, double)> callback) override;
 
   void UpdateBrowserControlsState(blink::WebBrowserControlsState constraints,
                                   blink::WebBrowserControlsState current,
                                   bool animate) override;
   void SetBrowserControlsHeight(float height, bool shrink) override;
   void SetBrowserControlsShownRatio(float) override;
   // TODO(ianwen): Move this method to WebLayerTreeView and implement main
   // thread scrolling.
   virtual void setBottomControlsHeight(float height);
 
@@ skipping 58 matching lines @@
   blink::WebLayoutAndPaintAsyncCallback* layout_and_paint_async_callback_;
 
   cc::FrameSinkId frame_sink_id_;
 
   base::WeakPtrFactory<RenderWidgetCompositor> weak_factory_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_GPU_RENDER_WIDGET_COMPOSITOR_H_