cc : Use screen space transform for computing translation

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 <stddef.h>
 
 #include <memory>
@@ skipping 139 matching lines @@
   // other id; then the nearest ancestor of node id1 whose id is smaller than
   // id2 is the lowest common ancestor of the pair of nodes, and the transform
   // from this lowest common ancestor to node id2 is only a 2d translation.
   bool ComputeTransform(int source_id,
                         int dest_id,
                         gfx::Transform* transform) const;
 
   // Computes the change of basis transform from node |source_id| to |dest_id|.
   // This is used by scroll children to compute transform from their scroll
   // parent space (source) to their parent space (destination) and it can atmost
-  // be a translation.
+  // be a translation. This function assumes that the path from source to
+  // destination has only translations. So, this function should not be used
+  // when there can be intermediate 3d transforms but the end result is a
+  // translation.
   bool ComputeTranslation(int source_id,
                           int dest_id,
                           gfx::Transform* transform) const;
 
   void ResetChangeTracking();
   // Updates the parent, target, and screen space transforms and snapping.
   void UpdateTransforms(int id);
   void UpdateTransformChanged(TransformNode* node,
                               TransformNode* parent_node,
                               TransformNode* source_node);
@@ skipping 435 matching lines @@
   gfx::Vector2dF inner_viewport_container_bounds_delta_;
   gfx::Vector2dF outer_viewport_container_bounds_delta_;
   gfx::Vector2dF inner_viewport_scroll_bounds_delta_;
 
   PropertyTreesCachedData cached_data_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_