[not for review - epic CL] Adding Elastic+Momentum+Layered scrolling to views::ScrollView

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index ccbd9e7393ee42373e8fea65dfebf6b39727e481..d37a28bbb40bf8de798b4f6ec8e114a555639c5a 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -199,7 +199,9 @@ class CC_EXPORT LayerTreeHostImpl
   bool DoTouchEventsBlockScrollAt(const gfx::Point& viewport_port) override;
   std::unique_ptr<SwapPromiseMonitor> CreateLatencyInfoSwapPromiseMonitor(
       ui::LatencyInfo* latency) override;
-  ScrollElasticityHelper* CreateScrollElasticityHelper() override;
+  ScrollElasticityHelper* CreateRootScrollElasticityHelper() override;
+  base::WeakPtr<ScrollElasticityHelper>
+  ScrollElasticityHelperForScrollingLayer() override;
   bool GetScrollOffsetForLayer(int layer_id,
                                gfx::ScrollOffset* offset) override;
   bool ScrollLayerTo(int layer_id, const gfx::ScrollOffset& offset) override;
@@ -326,6 +328,10 @@ class CC_EXPORT LayerTreeHostImpl
   ScrollbarAnimationController* ScrollbarAnimationControllerForId(
       int scroll_layer_id) const;
 
+  void RegisterScrollElasticityHelper(int scroll_layer_id);
+  void UnregisterScrollElasticityHelper(int scroll_layer_id);
+  ScrollElasticityHelper* ScrollElasticityHelperForId(int scroll_layer_id);
+
   DrawMode GetDrawMode() const;
 
   // Viewport size in draw space: this size is in physical pixels and is used
@@ -753,7 +759,7 @@ class CC_EXPORT LayerTreeHostImpl
 
   // An object to implement the ScrollElasticityHelper interface and
   // hold all state related to elasticity. May be NULL if never requested.
-  std::unique_ptr<ScrollElasticityHelper> scroll_elasticity_helper_;
+  std::unique_ptr<ScrollElasticityHelper> root_scroll_elasticity_helper_;
 
   bool tile_priorities_dirty_;
 
@@ -809,6 +815,10 @@ class CC_EXPORT LayerTreeHostImpl
   std::unordered_map<int, std::unique_ptr<ScrollbarAnimationController>>
       scrollbar_animation_controllers_;
 
+  // Map from scroll layer ID to elasticity helper.
+  std::unordered_map<int, std::unique_ptr<ScrollElasticityHelper>>
+      scroll_elasticity_helpers_;
+
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;
   MicroBenchmarkControllerImpl micro_benchmark_controller_;
   std::unique_ptr<SynchronousTaskGraphRunner>