MediaQueryListListener is the only client of ScriptCallback and it still uses VM-specific code guarded with #if USE(JSC). We should provide JSC and V8 implementations of MediaQueryListListener and get rid of ScriptCallback entirely. I'm not sure if it is a problem but no connection between MediaQueryList and MediaQueryListListener is reported to the V8 garbage collector(unlike references between DOM node wrapper and its event listeners).
Closing this since r260243 removes MediaQueryListListener, and ScriptCallback is long gone.