Mozart: Ensure time always runs forward.

services/gfx/compositor/scene_state.cc

Index: services/gfx/compositor/scene_state.cc
diff --git a/services/gfx/compositor/scene_state.cc b/services/gfx/compositor/scene_state.cc
index 319d4ed4700b822935727e5820525d2a6f1d9621..a5dab8272d4a95ad4ffdfe8d29867dbf86c7be03 100644
--- a/services/gfx/compositor/scene_state.cc
+++ b/services/gfx/compositor/scene_state.cc
@@ -10,7 +10,9 @@ 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