WIP: Surface Synchronization System

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 00fa09fd35f5f5fbf979df92053e179078df6b4e..df204791f9dc5ee8bc6d3889236b1d98526218e7 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1570,8 +1570,14 @@ CompositorFrameMetadata LayerTreeHostImpl::MakeCompositorFrameMetadata() const {
   }
 
   for (LayerImpl* surface_layer : active_tree_->SurfaceLayers()) {
+    SurfaceLayerImpl* surface_layer_impl =
+        static_cast<SurfaceLayerImpl*>(surface_layer);
     metadata.referenced_surfaces.push_back(
-        static_cast<SurfaceLayerImpl*>(surface_layer)->surface_info().id());
+        surface_layer_impl->primary_surface_info().id());
+    if (surface_layer_impl->fallback_surface_info().id().is_valid()) {
+      metadata.referenced_surfaces.push_back(
+          surface_layer_impl->fallback_surface_info().id());
+    }
   }
   if (!InnerViewportScrollLayer())
     return metadata;