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 d8fedafdd6fcc3463249edfe4cb873fb123bf4da..4a45f2c34c2b60f871d125c56c35bfbb9c513c12 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -59,11 +59,8 @@ 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);
 
@@ -141,6 +138,10 @@ class CC_SURFACES_EXPORT SurfaceManager {
                                 const FrameSinkId& frame_sink_id);
   void UnregisterBeginFrameSource(BeginFrameSource* source);
 
+  // Returns a stable BeginFrameSource that forwards BeginFrames from the first
+  // available BeginFrameSource.
+  BeginFrameSource* GetPrimaryBeginFrameSource();
+
   // Register a relationship between two namespaces.  This relationship means
   // that surfaces from the child namespace will be displayed in the parent.
   // Children are allowed to use any begin frame source that their parent can
@@ -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_;