Remove GpuHostMsg_FrameDrawn and replace with client channel swap ack

cc/output/output_surface.cc

Index: cc/output/output_surface.cc
diff --git a/cc/output/output_surface.cc b/cc/output/output_surface.cc
index 2dd5a213551ae3783d2521e4ad9074783c26b111..f0cbf5da205503f7244be7dad95744850d4392f7 100644
--- a/cc/output/output_surface.cc
+++ b/cc/output/output_surface.cc
@@ -223,6 +223,8 @@ void OutputSurface::BindFramebuffer() {
 }
 
 void OutputSurface::SwapBuffers(CompositorFrame* frame) {
+  // TODO(sievers): Make OutputSurface::SwapBuffers() pure virtual.
+  // The implementation here is used for tests only.
   if (frame->software_frame_data) {
     PostSwapBuffersComplete();
     client_->DidSwapBuffers();