cc: Make OutputSurface::SwapBuffers pure virtual

cc/test/fake_output_surface.h

Index: cc/test/fake_output_surface.h
diff --git a/cc/test/fake_output_surface.h b/cc/test/fake_output_surface.h
index 306879bbc340091f193f7cbc6bc72dc2a2515832..37c0c54a39c5a31351f1bb4f490463e7e2170c81 100644
--- a/cc/test/fake_output_surface.h
+++ b/cc/test/fake_output_surface.h
@@ -119,6 +119,10 @@ class FakeOutputSurface : public OutputSurface {
   void SetMemoryPolicyToSetAtBind(
       scoped_ptr<ManagedMemoryPolicy> memory_policy_to_set_at_bind);
 
+  gfx::Rect last_swap_rect() const {
+    return last_swap_rect_;
+  }
+
  protected:
   FakeOutputSurface(
       scoped_refptr<ContextProvider> context_provider,
@@ -142,6 +146,7 @@ class FakeOutputSurface : public OutputSurface {
   bool has_external_stencil_test_;
   TransferableResourceArray resources_held_by_parent_;
   scoped_ptr<ManagedMemoryPolicy> memory_policy_to_set_at_bind_;
+  gfx::Rect last_swap_rect_;
 
   base::WeakPtrFactory<FakeOutputSurface> fake_weak_ptr_factory_;
 };