Clarify the TODO for FrameRequestCallback

Source/core/dom/FrameRequestCallback.idl

Index: Source/core/dom/FrameRequestCallback.idl
diff --git a/Source/core/dom/FrameRequestCallback.idl b/Source/core/dom/FrameRequestCallback.idl
index b65b637c4749cc1a2b6cb7883b396e797635cb8d..ddf7cdab9650bead0365b011a54e29ce7693d93f 100644
--- a/Source/core/dom/FrameRequestCallback.idl
+++ b/Source/core/dom/FrameRequestCallback.idl
@@ -28,6 +28,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+// TODO(philipj): FrameRequestCallback should not be a callback interface.
+// https://www.w3.org/Bugs/Public/show_bug.cgi?id=28152
 callback interface FrameRequestCallback {
     // highResTime is passed as high resolution timestamp, see
     // http://www.w3.org/TR/hr-time/ for details.