cc: Allow the main thread to cancel commits

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/json/json_writer.h"
@@ skipping 1161 matching lines @@
 
 static void LayerTreeHostImplDidBeginTracingCallback(LayerImpl* layer) {
   layer->DidBeginTracing();
 }
 
 void LayerTreeHostImpl::DrawLayers(FrameData* frame,
                                    base::TimeTicks frame_begin_time) {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::DrawLayers");
   DCHECK(CanDraw());
 
-  if (frame->has_no_damage)
+  if (frame->has_no_damage) {
+    TRACE_EVENT0("cc", "EarlyOut_NoDamage");
     return;
+  }
 
   DCHECK(!frame->render_passes.empty());
 
   fps_counter_->SaveTimeStamp(frame_begin_time);
 
   rendering_stats_instrumentation_->SetScreenFrameCount(
       fps_counter_->current_frame_number());
   rendering_stats_instrumentation_->SetDroppedFrameCount(
       fps_counter_->dropped_frame_count());
 
@@ skipping 1202 matching lines @@
   if (LayerTreeDebugState::Equal(debug_state_, new_debug_state))
     return;
   if (debug_state_.continuous_painting != new_debug_state.continuous_painting)
     paint_time_counter_->ClearHistory();
 
   debug_state_ = new_debug_state;
   SetFullRootLayerDamage();
 }
 
 }  // namespace cc