Fix scroll position adjustment for fixed-position layers

cc/trees/property_tree.h

 // 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.
 
 #ifndef CC_TREES_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 69 matching lines @@
 
   bool needs_sublayer_scale;
   // This is used as a fallback when we either cannot adjust raster scale or if
   // the raster scale cannot be extracted from the screen space transform.
   float layer_scale_factor;
   gfx::Vector2dF sublayer_scale;
 
   // TODO(vollick): will be moved when accelerated effects are implemented.
   gfx::Vector2dF scroll_offset;
 
+  // We scroll snap where possible, but this has an effect on scroll
+  // compensation: the snap is yet more scrolling that must be compensated for.
+  // This value stores the snapped amount for this purpose.
+  gfx::Vector2dF scroll_snap;
+
   void set_to_parent(const gfx::Transform& transform) {
     to_parent = transform;
     is_invertible = to_parent.IsInvertible();
   }
 };
 
 typedef TreeNode<TransformNodeData> TransformNode;
 
 struct CC_EXPORT ClipNodeData {
   ClipNodeData();
@@ skipping 88 matching lines @@
   void UpdateSnapping(TransformNode* node);
 };
 
 class CC_EXPORT ClipTree final : public PropertyTree<ClipNode> {};
 
 class CC_EXPORT OpacityTree final : public PropertyTree<OpacityNode> {};
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_