implement the logic to set tile priorities based on current matrix

cc/picture_layer_impl.h

Index: cc/picture_layer_impl.h
diff --git a/cc/picture_layer_impl.h b/cc/picture_layer_impl.h
index 30b88d1760e6deb6a0a2ad77912d4f5e0a8d0771..7bc20188297666748e3d7ec50a3dd10751f44563 100644
--- a/cc/picture_layer_impl.h
+++ b/cc/picture_layer_impl.h
@@ -44,6 +44,13 @@ protected:
   PictureLayerTilingSet tilings_;
   scoped_refptr<PicturePileImpl> pile_;
 
+  gfx::Transform last_screen_space_transform_;
+  double last_update_time_;
+  gfx::Size last_bounds_;
+  gfx::Size last_content_bounds_;
+  float last_content_scale_x_;
+  float last_content_scale_y_;
+
   friend class PictureLayer;
   DISALLOW_COPY_AND_ASSIGN(PictureLayerImpl);
 };