Cache viewport rect for tile priority in UpdateTiles

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index e68bbc36f086c7ac1c1d5e5c58b88e87f1adf95a..ed7b406748e1e4692566c3996835de0c9bc8e2a4 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -412,6 +412,9 @@ class CC_EXPORT LayerTreeImpl {
   float top_controls_delta_;
   float sent_top_controls_delta_;
 
+  // The cached viewport rect for tile priority since the last call of
+  // UpdateDrawProperties.
+  gfx::Rect viewport_rect_for_tile_priority_;
   scoped_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private: