Fix scroll chaining for non-descendants of root scroller.

cc/layers/viewport.cc

 // Copyright 2015 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/layers/viewport.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "cc/input/top_controls_manager.h"
 #include "cc/trees/layer_tree_host_impl.h"
@@ skipping 177 matching lines @@
 
   Pan(move);
 }
 
 void Viewport::PinchEnd() {
   pinch_anchor_adjustment_ = gfx::Vector2d();
   pinch_zoom_active_ = false;
 }
 
 LayerImpl* Viewport::MainScrollLayer() const {
-  return InnerScrollLayer();
+  return OuterScrollLayer();
 }
 
 gfx::Vector2dF Viewport::ScrollTopControls(const gfx::Vector2dF& delta) {
   gfx::Vector2dF excess_delta =
       host_impl_->top_controls_manager()->ScrollBy(delta);
 
   return delta - excess_delta;
 }
 
 bool Viewport::ShouldTopControlsConsumeScroll(
@@ skipping 46 matching lines @@
 
 LayerImpl* Viewport::InnerScrollLayer() const {
   return host_impl_->InnerViewportScrollLayer();
 }
 
 LayerImpl* Viewport::OuterScrollLayer() const {
   return host_impl_->OuterViewportScrollLayer();
 }
 
 }  // namespace cc