cc: Make OutputSurface::SwapBuffers pure virtual

cc/output/output_surface.h

 // Copyright 2012 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_H_
 #define CC_OUTPUT_OUTPUT_SURFACE_H_
 
 #include <deque>
 
 #include "base/basictypes.h"
@@ skipping 95 matching lines @@
   virtual void EnsureBackbuffer();
   virtual void DiscardBackbuffer();
 
   virtual void Reshape(const gfx::Size& size, float scale_factor);
   virtual gfx::Size SurfaceSize() const;
 
   virtual void BindFramebuffer();
 
   // The implementation may destroy or steal the contents of the CompositorFrame
   // passed in (though it will not take ownership of the CompositorFrame
-  // itself).
-  virtual void SwapBuffers(CompositorFrame* frame);
+  // itself). For successful swaps, the implementation must call
+  // OutputSurfaceClient::DidSwapBuffers() and eventually
+  // DidSwapBuffersComplete().
+  virtual void SwapBuffers(CompositorFrame* frame) = 0;
   virtual void OnSwapBuffersComplete();
 
   // Notifies frame-rate smoothness preference. If true, all non-critical
   // processing should be stopped, or lowered in priority.
   virtual void UpdateSmoothnessTakesPriority(bool prefer_smoothness) {}
 
   // Requests a BeginFrame notification from the output surface. The
   // notification will be delivered by calling
   // OutputSurfaceClient::BeginFrame until the callback is disabled.
   virtual void SetNeedsBeginFrame(bool enable) {}
@@ skipping 47 matching lines @@
   bool external_stencil_test_enabled_;
 
   base::WeakPtrFactory<OutputSurface> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(OutputSurface);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_OUTPUT_SURFACE_H_