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

cc/trees/draw_property_utils.cc

 // Copyright 2014 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/draw_property_utils.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 1578 matching lines @@
                                 page_scale_factor, device_scale_factor,
                                 device_transform);
 }
 
 template <typename LayerType>
 static void UpdateElasticOverscrollInternal(
     PropertyTrees* property_trees,
     const LayerType* overscroll_elasticity_layer,
     const gfx::Vector2dF& elastic_overscroll) {
   if (!overscroll_elasticity_layer) {
-    DCHECK(elastic_overscroll.IsZero());
+    // When only the viewport has elasticity, elastic_overscroll should be zero
+    // here, but for now elasticity doesn't bubble up, only down. So, that won't
+    // be the case if a sublayer has its own overscroll.
+    // TODO(tapted): Give the viewport overscroll and completely remove
+    // overscroll_elasticity_layer.
     return;
   }
 
   TransformNode* node = property_trees->transform_tree.Node(
       overscroll_elasticity_layer->transform_tree_index());
-  if (node->scroll_offset == gfx::ScrollOffset(elastic_overscroll))
+  if (node->scroll_offset_with_overscroll ==
+      gfx::ScrollOffset(elastic_overscroll))
     return;
 
-  node->scroll_offset = gfx::ScrollOffset(elastic_overscroll);
+  node->scroll_offset_with_overscroll = gfx::ScrollOffset(elastic_overscroll);
   node->needs_local_transform_update = true;
   property_trees->transform_tree.set_needs_update(true);
 }
 
 void UpdateElasticOverscroll(PropertyTrees* property_trees,
                              const LayerImpl* overscroll_elasticity_layer,
                              const gfx::Vector2dF& elastic_overscroll) {
   UpdateElasticOverscrollInternal(property_trees, overscroll_elasticity_layer,
                                   elastic_overscroll);
 }
 
 void UpdateElasticOverscroll(PropertyTrees* property_trees,
                              const Layer* overscroll_elasticity_layer,
                              const gfx::Vector2dF& elastic_overscroll) {
   UpdateElasticOverscrollInternal(property_trees, overscroll_elasticity_layer,
                                   elastic_overscroll);
 }
 
 }  // namespace draw_property_utils
 
 }  // namespace cc