Fix scroll chaining in CC for non-default RootScroller.

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 3085 matching lines @@
   // TODO(majidvp): in Blink we compute scroll chain only at scroll begin which
   // is not the case here. We eventually want to have the same behaviour on both
   // sides but it may become a non issue if we get rid of scroll chaining (see
   // crbug.com/526462)
   std::list<const ScrollNode*> current_scroll_chain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
   if (scroll_node) {
     for (; scroll_tree.parent(scroll_node);
          scroll_node = scroll_tree.parent(scroll_node)) {
-      // Skip the outer viewport scroll layer so that we try to scroll the
-      // viewport only once. i.e. The inner viewport layer represents the
-      // viewport.
-      if (!scroll_node->scrollable ||
-          scroll_node->is_outer_viewport_scroll_layer)
+      if (scroll_node->is_outer_viewport_scroll_layer) {
+        // Don't chain scrolls past the outer viewport scroll layer. Once we
+        // reach that, we should scroll the viewport, which is represented by
+        // the inner viewport scroll layer.
+        ScrollNode* inner_viewport_scroll_node =
+            scroll_tree.Node(InnerViewportScrollLayer()->scroll_tree_index());
+        current_scroll_chain.push_front(inner_viewport_scroll_node);
+        break;
+      }
+
+      if (!scroll_node->scrollable)
         continue;
+
       current_scroll_chain.push_front(scroll_node);
     }
   }
   scroll_state->set_scroll_chain_and_layer_tree(current_scroll_chain,
                                                 active_tree());
   scroll_state->DistributeToScrollChainDescendant();
 }
 
 InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     ScrollState* scroll_state) {
@@ skipping 979 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