Cache image data objects.

ppapi/thunk/ppb_graphics_2d_api.h

Index: ppapi/thunk/ppb_graphics_2d_api.h
diff --git a/ppapi/thunk/ppb_graphics_2d_api.h b/ppapi/thunk/ppb_graphics_2d_api.h
index 80c8c78f272fd51aa129dba2ddfff6c4bba149f7..6f14b936fde195b6e0ad69a599af7ddb7aeac03a 100644
--- a/ppapi/thunk/ppb_graphics_2d_api.h
+++ b/ppapi/thunk/ppb_graphics_2d_api.h
@@ -27,9 +27,20 @@ class PPB_Graphics2D_API {
   virtual void Scroll(const PP_Rect* clip_rect,
                       const PP_Point* amount) = 0;
   virtual void ReplaceContents(PP_Resource image_data) = 0;
-  virtual int32_t Flush(scoped_refptr<TrackedCallback> callback) = 0;
   virtual bool SetScale(float scale) = 0;
   virtual float GetScale() = 0;
+
+  // When |old_image_data| is non-null and the flush is executing a replace
+  // contents (which leaves the "old" ImageData unowned), the resource ID of
+  // the old image data will be placed into |*old_image_data| synchronously
+  // (not when the flush callback completes).
+  //
+  // When this happens, a reference to this resource will be transferred to the
+  // caller. If there is no replace contents operation, old_image_data will be
+  // ignored. If |*old_image_data| is null, then the old image data will be
+  // destroyed if there was one.
+  virtual int32_t Flush(scoped_refptr<TrackedCallback> callback,
+                        PP_Resource* old_image_data) = 0;
 };
 
 }  // namespace thunk