Mozart: Directly associate the renderer with the view tree.

services/gfx/compositor/renderer_state.cc

Index: services/gfx/compositor/renderer_state.cc
diff --git a/services/gfx/compositor/renderer_state.cc b/services/gfx/compositor/renderer_state.cc
index 28ba6870bf9bfcdf18781cc332713d73dc5fe7f5..37bf319aac87a7a128a4766e76395e675b95cf25 100644
--- a/services/gfx/compositor/renderer_state.cc
+++ b/services/gfx/compositor/renderer_state.cc
@@ -18,7 +18,6 @@ bool RendererState::SetRootScene(SceneState* scene,
                                  uint32_t version,
                                  const mojo::Rect& viewport) {
   DCHECK(scene);
-  DCHECK(version);
 
   if (root_scene_ != scene || root_scene_version_ != version ||
       !root_scene_viewport_.Equals(viewport)) {
@@ -31,6 +30,15 @@ bool RendererState::SetRootScene(SceneState* scene,
   return false;
 }
 
+bool RendererState::ResetRootScene() {
+  if (root_scene_) {
+    root_scene_ = nullptr;
+    SetSnapshot(nullptr);
+    return true;
+  }
+  return false;
+}
+
 bool RendererState::SetSnapshot(std::unique_ptr<Snapshot> snapshot) {
   snapshot_ = std::move(snapshot);
   if (snapshot_ && snapshot_->valid()) {