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

cc/surfaces/surface_manager.h

Index: cc/surfaces/surface_manager.h
diff --git a/cc/surfaces/surface_manager.h b/cc/surfaces/surface_manager.h
index 293f4c8af80a037f6c3660f08cd02cb4b640e161..cb7fb15d5cd2e448f306883e62bea2c1e4606f0b 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -59,14 +59,15 @@ class CC_SURFACES_EXPORT SurfaceManager {
   std::string SurfaceReferencesToString();
 #endif
 
-  void SetDependencyTracker(
-      std::unique_ptr<SurfaceDependencyTracker> dependency_tracker);
-  SurfaceDependencyTracker* dependency_tracker() {
-    return dependency_tracker_.get();
-  }
+  void SetDependencyTracker(SurfaceDependencyTracker* dependency_tracker);
+  SurfaceDependencyTracker* dependency_tracker() { return dependency_tracker_; }
 
   void RequestSurfaceResolution(Surface* pending_surface);
 
+  // Creates a BeginFrameSource that tracks the first registered
+  // BeginFrameSource.
+  std::unique_ptr<BeginFrameSource> CreatePrimaryBeginFrameSource();
+
   std::unique_ptr<Surface> CreateSurface(
       base::WeakPtr<CompositorFrameSinkSupport> compositor_frame_sink_support,
       const LocalSurfaceId& local_surface_id);
@@ -288,7 +289,7 @@ class CC_SURFACES_EXPORT SurfaceManager {
   std::unordered_map<FrameSinkId, std::vector<LocalSurfaceId>, FrameSinkIdHash>
       temporary_reference_ranges_;
 
-  std::unique_ptr<SurfaceDependencyTracker> dependency_tracker_;
+  SurfaceDependencyTracker* dependency_tracker_ = nullptr;
 
   base::WeakPtrFactory<SurfaceManager> weak_factory_;