Update from https://crrev.com/304715

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 <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 2549 matching lines @@
     return InputHandlerScrollResult();
 
   gfx::Vector2dF pending_delta = scroll_delta;
   gfx::Vector2dF unused_root_delta;
   bool did_scroll_x = false;
   bool did_scroll_y = false;
   bool did_scroll_top_controls = false;
 
   bool consume_by_top_controls = ShouldTopControlsConsumeScroll(scroll_delta);
 
-  for (LayerImpl* layer_impl = CurrentlyScrollingLayer();
+  // There's an edge case where the outer viewport isn't scrollable when the
+  // scroll starts, however, as the top controls show the outer viewport becomes
+  // scrollable. Therefore, always try scrolling the outer viewport before the
+  // inner.
+  // TODO(bokan): Move the top controls logic out of the loop since the scroll
+  // that causes the outer viewport to become scrollable will still be applied
+  // to the inner viewport.
+  LayerImpl* start_layer = CurrentlyScrollingLayer();
+  if (start_layer == InnerViewportScrollLayer() && OuterViewportScrollLayer())
+      start_layer = OuterViewportScrollLayer();
+
+  for (LayerImpl* layer_impl = start_layer;
        layer_impl;
        layer_impl = layer_impl->parent()) {
     if (!layer_impl->scrollable())
       continue;
 
     if (layer_impl == InnerViewportScrollLayer() ||
         layer_impl == OuterViewportScrollLayer()) {
       if (consume_by_top_controls) {
         gfx::Vector2dF excess_delta =
             top_controls_manager_->ScrollBy(pending_delta);
@@ skipping 850 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc