Preserve FrameSinkSourceMapping nodes that have path to root

cc/surfaces/surface_manager.cc

Index: cc/surfaces/surface_manager.cc
diff --git a/cc/surfaces/surface_manager.cc b/cc/surfaces/surface_manager.cc
index 753191d7460be547e751b117afe584ad3734c89b..d5638be959559b1ecf4f43b9233183a678f63310 100644
--- a/cc/surfaces/surface_manager.cc
+++ b/cc/surfaces/surface_manager.cc
@@ -582,7 +582,8 @@ void SurfaceManager::UnregisterFrameSinkHierarchy(
   // The SurfaceFactoryClient and hierarchy can be registered/unregistered
   // in either order, so empty frame_sink_source_map entries need to be
   // checked when removing either clients or relationships.
-  if (!iter->second.has_children() && !clients_.count(parent_frame_sink_id)) {
+  if (!iter->second.has_children() && !clients_.count(parent_frame_sink_id) &&
+      !iter->second.source) {
     frame_sink_source_map_.erase(iter);
     return;
   }