Clarify the TODO for FrameRequestCallback

Source/core/dom/FrameRequestCallback.idl

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@@ skipping 10 matching lines @@
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+// 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.
     void handleEvent(double highResTime);
 };