cc::SurfaceDependencyTracker should not crash when a Display goes away

cc/surfaces/surface_manager.cc

Index: cc/surfaces/surface_manager.cc
diff --git a/cc/surfaces/surface_manager.cc b/cc/surfaces/surface_manager.cc
index cbdb78c5ea4a061887bcfc14fb2f627904fafd3f..019d9832770548e14afd5898c863ef68ec488f33 100644
--- a/cc/surfaces/surface_manager.cc
+++ b/cc/surfaces/surface_manager.cc
@@ -65,8 +65,8 @@ std::string SurfaceManager::SurfaceReferencesToString() {
 #endif
 
 void SurfaceManager::SetDependencyTracker(
-    std::unique_ptr<SurfaceDependencyTracker> dependency_tracker) {
-  dependency_tracker_ = std::move(dependency_tracker);
+    SurfaceDependencyTracker* dependency_tracker) {
+  dependency_tracker_ = dependency_tracker;
 }
 
 void SurfaceManager::RequestSurfaceResolution(Surface* pending_surface) {
@@ -74,6 +74,11 @@ void SurfaceManager::RequestSurfaceResolution(Surface* pending_surface) {
     dependency_tracker_->RequestSurfaceResolution(pending_surface);
 }
 
+std::unique_ptr<BeginFrameSource>
+SurfaceManager::CreatePrimaryBeginFrameSource() {
+  return framesink_manager_.CreatePrimaryBeginFrameSource();
+}
+
 std::unique_ptr<Surface> SurfaceManager::CreateSurface(
     base::WeakPtr<CompositorFrameSinkSupport> compositor_frame_sink_support,
     const LocalSurfaceId& local_surface_id) {