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

cc/surfaces/surface_synchronization_unittest.cc

Index: cc/surfaces/surface_synchronization_unittest.cc
diff --git a/cc/surfaces/surface_synchronization_unittest.cc b/cc/surfaces/surface_synchronization_unittest.cc
index ae7d748770d41d0a65eb758172f4511406dbaffe..4c0f12d5ca7c6bf475b8e14fb023348d9288b3c1 100644
--- a/cc/surfaces/surface_synchronization_unittest.cc
+++ b/cc/surfaces/surface_synchronization_unittest.cc
@@ -107,9 +107,9 @@ class SurfaceSynchronizationTest : public testing::Test,
 
     begin_frame_source_ =
         base::MakeUnique<FakeExternalBeginFrameSource>(0.f, false);
-    surface_manager_.SetDependencyTracker(
-        base::MakeUnique<SurfaceDependencyTracker>(&surface_manager_,
-                                                   begin_frame_source_.get()));
+    dependency_tracker_ = base::MakeUnique<SurfaceDependencyTracker>(
+        &surface_manager_, begin_frame_source_.get());
+    surface_manager_.SetDependencyTracker(dependency_tracker_.get());
     surface_manager_.AddObserver(this);
     supports_.push_back(CompositorFrameSinkSupport::Create(
         &support_client_, &surface_manager_, kDisplayFrameSink, kIsRoot,
@@ -137,6 +137,8 @@ class SurfaceSynchronizationTest : public testing::Test,
     surface_manager_.SetDependencyTracker(nullptr);
     surface_manager_.UnregisterBeginFrameSource(begin_frame_source_.get());
 
+    dependency_tracker_.reset();
+
     // SurfaceDependencyTracker depends on this BeginFrameSource and so it must
     // be destroyed AFTER the dependency tracker is destroyed.
     begin_frame_source_.reset();
@@ -163,6 +165,7 @@ class SurfaceSynchronizationTest : public testing::Test,
   base::flat_set<SurfaceId> damaged_surfaces_;
   SurfaceManager surface_manager_;
   std::unique_ptr<FakeExternalBeginFrameSource> begin_frame_source_;
+  std::unique_ptr<SurfaceDependencyTracker> dependency_tracker_;
   std::vector<std::unique_ptr<CompositorFrameSinkSupport>> supports_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceSynchronizationTest);