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

cc/surfaces/frame_sink_manager.h

Index: cc/surfaces/frame_sink_manager.h
diff --git a/cc/surfaces/frame_sink_manager.h b/cc/surfaces/frame_sink_manager.h
index 0d54a8e3d8f002356887f22bc78f6e512d041401..4d68dc4237dcf8123a2f70a1213aefdff33fec47 100644
--- a/cc/surfaces/frame_sink_manager.h
+++ b/cc/surfaces/frame_sink_manager.h
@@ -13,7 +13,9 @@
 
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "cc/surfaces/frame_sink_id.h"
+#include "cc/surfaces/frame_sink_observer.h"
 #include "cc/surfaces/surfaces_export.h"
 
 namespace cc {
@@ -50,12 +52,23 @@ class CC_SURFACES_EXPORT FrameSinkManager {
                                       FrameSinkManagerClient* client);
   void UnregisterFrameSinkManagerClient(const FrameSinkId& frame_sink_id);
 
+  // Creates a BeginFrameSource that tracks the first registered
+  // BeginFrameSource.
+  std::unique_ptr<BeginFrameSource> CreatePrimaryBeginFrameSource();
+
+  void AddObserver(FrameSinkObserver* obs) { observer_list_.AddObserver(obs); }
+
+  void RemoveObserver(FrameSinkObserver* obs) {
+    observer_list_.RemoveObserver(obs);
+  }
+
   // Associates a |source| with a particular framesink.  That framesink and
   // any children of that framesink with valid clients can potentially use
   // that |source|.
   void RegisterBeginFrameSource(BeginFrameSource* source,
                                 const FrameSinkId& frame_sink_id);
   void UnregisterBeginFrameSource(BeginFrameSource* source);
+  BeginFrameSource* GetPrimaryBeginFrameSource();
 
   // Register a relationship between two framesinks.  This relationship means
   // that surfaces from the child framesik will be displayed in the parent.
@@ -99,18 +112,20 @@ class CC_SURFACES_EXPORT FrameSinkManager {
     ~FrameSinkSourceMapping();
     bool has_children() const { return !children.empty(); }
     // The currently assigned begin frame source for this client.
-    BeginFrameSource* source;
+    BeginFrameSource* source = nullptr;
     // This represents a dag of parent -> children mapping.
     std::vector<FrameSinkId> children;
   };
 
+  base::ObserverList<FrameSinkObserver> observer_list_;
+
   std::unordered_map<FrameSinkId, FrameSinkManagerClient*, FrameSinkIdHash>
       clients_;
 
   std::unordered_map<FrameSinkId, FrameSinkSourceMapping, FrameSinkIdHash>
       frame_sink_source_map_;
 
-  // Set of which sources are registered to which frmesinks.  Any child
+  // Set of BeginFrameSource along with associated FrameSinkIds. Any child
   // that is implicitly using this framesink must be reachable by the
   // parent in the dag.
   std::unordered_map<BeginFrameSource*, FrameSinkId> registered_sources_;