Retain references to surfaces from both active AND pending CompositorFrames

cc/surfaces/compositor_frame_sink_support.h

Index: cc/surfaces/compositor_frame_sink_support.h
diff --git a/cc/surfaces/compositor_frame_sink_support.h b/cc/surfaces/compositor_frame_sink_support.h
index cfdd0a2539f4827ed51b505cdab76feaf74b06b8..6bf64d145665756fd53c95ed92402d2f88adbb0e 100644
--- a/cc/surfaces/compositor_frame_sink_support.h
+++ b/cc/surfaces/compositor_frame_sink_support.h
@@ -46,6 +46,10 @@ class CC_SURFACES_EXPORT CompositorFrameSinkSupport
     return surface_factory_.current_surface_for_testing();
   }
 
+  const ReferencedSurfaceTracker& ReferenceTrackerForTesting() const {
+    return reference_tracker_;
+  }
+
   void EvictFrame();
   void SetNeedsBeginFrame(bool needs_begin_frame);
   void SubmitCompositorFrame(const LocalSurfaceId& local_surface_id,