Fix scroll chaining for non-descendants of root scroller.

cc/trees/layer_tree_host_impl.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 580 matching lines @@
                              const gfx::Vector2dF& scroll_amount,
                              base::TimeDelta delayed_by);
 
   void SetLayerTreeMutator(std::unique_ptr<LayerTreeMutator> mutator);
   LayerTreeMutator* mutator() { return mutator_.get(); }
 
   void ClearCurrentlyScrollingLayerForTesting() {
     ClearCurrentlyScrollingLayer();
   }
 
+  LayerImpl* ViewportMainScrollLayer();
+
  protected:
   LayerTreeHostImpl(
       const LayerTreeSettings& settings,
       LayerTreeHostImplClient* client,
       TaskRunnerProvider* task_runner_provider,
       RenderingStatsInstrumentation* rendering_stats_instrumentation,
       SharedBitmapManager* shared_bitmap_manager,
       gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager,
       TaskGraphRunner* task_graph_runner,
       std::unique_ptr<AnimationHost> animation_host,
@@ skipping 226 matching lines @@
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_