Mozart: Ensure time always runs forward.

services/gfx/compositor/scene_state.cc

 // Copyright 2015 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 "services/gfx/compositor/scene_state.h"
 
 namespace compositor {
 
 SceneState::SceneState(mojo::gfx::composition::SceneTokenPtr scene_token,
                        const std::string& label)
     : scene_token_(scene_token.Pass()),
       scene_def_(SceneLabel(scene_token_->value, label)),
-      weak_factory_(this) {}
+      weak_factory_(this) {
+  DCHECK(scene_token_);
+}
 
 SceneState::~SceneState() {
   // The scene implementation and all of its bindings must be destroyed
   // before any pending callbacks are dropped on the floor.
   scene_impl_.reset();
   pending_frame_callbacks_.clear();
 }
 
 void SceneState::AddSceneFrameCallback(const SceneFrameCallback& callback) {
   pending_frame_callbacks_.push_back(callback);
 }
 
 void SceneState::DispatchSceneFrameCallbacks(
     const mojo::gfx::composition::FrameInfo& frame_info) {
   for (auto& callback : pending_frame_callbacks_) {
     callback.Run(frame_info.Clone());
   }
   pending_frame_callbacks_.clear();
 }
 
 std::ostream& operator<<(std::ostream& os, SceneState* scene_state) {
   if (!scene_state)
     return os << "null";
   return os << scene_state->scene_def()->FormattedLabel();
 }
 
 }  // namespace compositor