cc: Update discardable image metadata generation to get PaintImages.

cc/paint/display_item_list.h

Index: cc/paint/display_item_list.h
diff --git a/cc/paint/display_item_list.h b/cc/paint/display_item_list.h
index 9d2c5b6b05aff27d18556395b3a8aa0602f44408..92c332fe8eddbafeaf76d5aa8d1c2169c063d093 100644
--- a/cc/paint/display_item_list.h
+++ b/cc/paint/display_item_list.h
@@ -164,6 +164,11 @@ class CC_PAINT_EXPORT DisplayItemList
     return items_.end();
   }
 
+  void GatherDiscardableImages(DiscardableImageStore* image_store) const;
+  const DiscardableImageMap& discardable_image_map_for_testing() const {
+    return image_map_;
+  }
+
   bool has_discardable_images() const { return has_discardable_images_; }
 
  private: