cc: Address comments for https://codereview.chromium.org/1287043002/

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 67fef2e01a9906b407e6eeae196e0c76ac27d48d..ae543a3e207deae1015580ce5e0d09d84f6aed96 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -219,6 +219,8 @@ void ThreadProxy::SetOutputSurface(scoped_ptr<OutputSurface> output_surface) {
 
 scoped_ptr<OutputSurface> ThreadProxy::ReleaseOutputSurface() {
   DCHECK(IsMainThread());
+  DCHECK(layer_tree_host()->output_surface_lost());
+
   DebugScopedSetMainThreadBlocked main_thread_blocked(this);
   CompletionEvent completion;
   scoped_ptr<OutputSurface> output_surface;
@@ -227,7 +229,7 @@ scoped_ptr<OutputSurface> ThreadProxy::ReleaseOutputSurface() {
       base::Bind(&ThreadProxy::ReleaseOutputSurfaceOnImplThread,
                  impl_thread_weak_ptr_, &completion, &output_surface));
   completion.Wait();
-  return output_surface.Pass();
+  return output_surface;
 }
 
 void ThreadProxy::DidInitializeOutputSurface(
@@ -1083,9 +1085,10 @@ void ThreadProxy::InitializeOutputSurfaceOnImplThread(
 void ThreadProxy::ReleaseOutputSurfaceOnImplThread(
     CompletionEvent* completion,
     scoped_ptr<OutputSurface>* output_surface) {
-  TRACE_EVENT0("cc", "ThreadProxy::ReleaseOutputSurfaceOnImplThread");
   DCHECK(IsImplThread());
 
+  // Unlike DidLoseOutputSurfaceOnImplThread, we don't need to call
+  // LayerTreeHost::DidLoseOutputSurface since it already knows.
   impl().scheduler->DidLoseOutputSurface();
   *output_surface = impl().layer_tree_host_impl->ReleaseOutputSurface();
   completion->Signal();