Made layout viewport scroll updates from compositor work like ordinary layers.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3352 matching lines @@
       callbacks->push_back(callback);
   }
   return callbacks;
 }
 
 static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
                                 LayerTreeImpl* tree_impl) {
   if (tree_impl->LayerListIsEmpty())
     return;
 
+  int inner_viewport_layer_id =
+      tree_impl->InnerViewportScrollLayer()
+          ? tree_impl->InnerViewportScrollLayer()->id()
+          : Layer::INVALID_ID;
+
   return tree_impl->property_trees()->scroll_tree.CollectScrollDeltas(
-      scroll_info);
+      scroll_info, inner_viewport_layer_id);
 }
 
 std::unique_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   std::unique_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
   CollectScrollDeltas(scroll_info.get(), active_tree_.get());
   scroll_info->page_scale_delta =
       active_tree_->page_scale_factor()->PullDeltaForMainThread();
   scroll_info->top_controls_delta =
       active_tree()->top_controls_shown_ratio()->PullDeltaForMainThread();
@@ skipping 641 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc