Refactor delegated scrolling in LayerImpl

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index b5f8aed2307ba159c5c091f123706c768324942f..33b8fe494aa2235159a997411c9d4e7d4b0e9b68 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -64,7 +64,7 @@ class LayerScrollOffsetDelegateProxy : public LayerImpl::ScrollOffsetDelegate {
   }
 
   void Update() const override {
-    layer_tree_impl_->UpdateScrollOffsetDelegate();
+    layer_tree_impl_->UpdateScrollOffsetDelegateIfNeeded();
   }
 
  private:
@@ -1070,25 +1070,30 @@ void LayerTreeImpl::SetRootLayerScrollOffsetDelegate(
       outer_viewport_scroll_layer_->RefreshFromScrollDelegate();
 
     if (inner_viewport_scroll_layer_)
-      UpdateScrollOffsetDelegate();
+      UpdateScrollOffsetDelegateIfNeeded();
   }
 }
 
 void LayerTreeImpl::OnRootLayerDelegatedScrollOffsetChanged() {
   DCHECK(root_layer_scroll_offset_delegate_);
-  if (inner_viewport_scroll_layer_) {
+  root_layer_scroll_offset_delegate_->SetBlockUpdates(true);
+  if (inner_viewport_scroll_layer_)
     inner_viewport_scroll_layer_->RefreshFromScrollDelegate();
-  }
-  if (outer_viewport_scroll_layer_) {
+
+  if (outer_viewport_scroll_layer_)
     outer_viewport_scroll_layer_->RefreshFromScrollDelegate();
-  }
-}
 
-void LayerTreeImpl::UpdateScrollOffsetDelegate() {
-  DCHECK(InnerViewportScrollLayer());
-  DCHECK(!OuterViewportScrollLayer() || outer_viewport_scroll_delegate_proxy_);
-  DCHECK(root_layer_scroll_offset_delegate_);
+  root_layer_scroll_offset_delegate_->SetBlockUpdates(false);
+  UpdateScrollOffsetDelegateIfNeeded();
+}
 
+void LayerTreeImpl::UpdateScrollOffsetDelegateIfNeeded() {
+  if (!root_layer_scroll_offset_delegate_)
+    return;
+  if (!InnerViewportScrollLayer() || !inner_viewport_scroll_delegate_proxy_)
+    return;
+  if (OuterViewportScrollLayer() && !outer_viewport_scroll_delegate_proxy_)
+    return;
   gfx::ScrollOffset offset =
       inner_viewport_scroll_delegate_proxy_->last_set_scroll_offset();