Only set a valid surface from html_viewer

mojo/services/html_viewer/weblayertreeview_impl.cc

 // Copyright 2014 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 "mojo/services/html_viewer/weblayertreeview_impl.h"
 
 #include "base/message_loop/message_loop_proxy.h"
 #include "cc/blink/web_layer_impl.h"
 #include "cc/layers/layer.h"
 #include "cc/output/begin_frame_args.h"
@@ skipping 83 matching lines @@
 }
 
 void WebLayerTreeViewImpl::ApplyViewportDeltas(
     const gfx::Vector2d& scroll_delta,
     float page_scale,
     float top_controls_delta) {
   widget_->applyViewportDeltas(scroll_delta, page_scale, top_controls_delta);
 }
 
 void WebLayerTreeViewImpl::RequestNewOutputSurface() {
-  layer_tree_host_->SetOutputSurface(output_surface_.Pass());
+  if (output_surface_.get())
+    layer_tree_host_->SetOutputSurface(output_surface_.Pass());
 }
 
 void WebLayerTreeViewImpl::DidFailToInitializeOutputSurface() {
   RequestNewOutputSurface();
 }
 
 void WebLayerTreeViewImpl::DidInitializeOutputSurface() {
 }
 
 void WebLayerTreeViewImpl::WillCommit() {
@@ skipping 122 matching lines @@
       base::Bind(&WebLayerTreeViewImpl::DidCreateSurfaceOnMainThread,
                  main_thread_bound_weak_ptr_,
                  id));
 }
 
 void WebLayerTreeViewImpl::DidCreateSurfaceOnMainThread(cc::SurfaceId id) {
   view_->SetSurfaceId(mojo::SurfaceId::From(id));
 }
 
 }  // namespace html_viewer