cc: Make OutputSurfaceFrame::sub_buffer_rect optional

cc/output/output_surface_frame.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 CC_OUTPUT_OUTPUT_SURFACE_FRAME_H_
 #define CC_OUTPUT_OUTPUT_SURFACE_FRAME_H_
 
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "cc/base/cc_export.h"
 #include "ui/events/latency_info.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace cc {
 
 // Metadata given to the OutputSurface for it to swap what was drawn and make
 // current frame visible.
 class CC_EXPORT OutputSurfaceFrame {
  public:
   OutputSurfaceFrame();
   OutputSurfaceFrame(OutputSurfaceFrame&& other);
   ~OutputSurfaceFrame();
 
   OutputSurfaceFrame& operator=(OutputSurfaceFrame&& other);
 
   gfx::Size size;
-  gfx::Rect sub_buffer_rect;
+  // Optional rect for partial or empty swap; if not provided, use regular swap.
+  base::Optional<gfx::Rect> sub_buffer_rect;
   std::vector<ui::LatencyInfo> latency_info;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(OutputSurfaceFrame);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_OUTPUT_SURFACE_FRAME_H_