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 9875d04d29debce79e941266f16c4eff507e1ed6..07d2338009404168851632729f0830b1ec739e79 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) {
@@ -431,6 +431,10 @@ void SurfaceManager::UnregisterBeginFrameSource(BeginFrameSource* source) {
   framesink_manager_.UnregisterBeginFrameSource(source);
 }
 
+BeginFrameSource* SurfaceManager::GetPrimaryBeginFrameSource() {
+  return framesink_manager_.GetPrimaryBeginFrameSource();
+}
+
 void SurfaceManager::RegisterFrameSinkHierarchy(
     const FrameSinkId& parent_frame_sink_id,
     const FrameSinkId& child_frame_sink_id) {