cc: Make OutputSurface::SwapBuffers pure virtual

cc/output/overlay_unittest.cc

Index: cc/output/overlay_unittest.cc
diff --git a/cc/output/overlay_unittest.cc b/cc/output/overlay_unittest.cc
index 698b42b0bfe89f6d19e520c6bac2579dfa787edc..9e837d57c0b83a02c3c4488b16fedd9ca7f6ef5d 100644
--- a/cc/output/overlay_unittest.cc
+++ b/cc/output/overlay_unittest.cc
@@ -102,11 +102,19 @@ class OverlayOutputSurface : public OutputSurface {
   explicit OverlayOutputSurface(scoped_refptr<ContextProvider> context_provider)
       : OutputSurface(context_provider) {}
 
+  // OutputSurface implementation
+  virtual void SwapBuffers(CompositorFrame* frame) override;
+
   void InitWithSingleOverlayValidator() {
     overlay_candidate_validator_.reset(new SingleOverlayValidator);
   }
 };
 
+void OverlayOutputSurface::SwapBuffers(CompositorFrame* frame) {
+  client_->DidSwapBuffers();
+  client_->DidSwapBuffersComplete();
+}
+
 scoped_ptr<RenderPass> CreateRenderPass() {
   RenderPassId id(1, 0);
   gfx::Rect output_rect(0, 0, 256, 256);