cc : Use screen space transform for computing translation

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 9bf1ed4430f418fb12d3c74b895135c4575d85ec..a97bad8e052ebcb973ce0b96dd8eba05c289a9a3 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -157,7 +157,10 @@ class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {
   // 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;