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

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index bcda921ff86dc86bf20feb23499c812330c2107d..61a963e0e1b563e8469bb3d88bdc29e4674ba80e 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -150,6 +150,9 @@ void SingleThreadProxy::RequestNewOutputSurface() {
 }
 
 scoped_ptr<OutputSurface> SingleThreadProxy::ReleaseOutputSurface() {
+  // |layer_tree_host_| should already be aware of this.
+  DCHECK(layer_tree_host_->output_surface_lost());
+
   if (scheduler_on_impl_thread_)
     scheduler_on_impl_thread_->DidLoseOutputSurface();
   return layer_tree_host_impl_->ReleaseOutputSurface();