Bug 998377 - Stop nulling out nsInProcessTabChildGlobal::mGlobal during DelayedDisconnect. r=smaug

This is already cycle-collected, so no extra work needs to happen.
Bug 998377 - Stop calling ReleaseWrapper in DelayedDisconnect(). r=smaug

AFAICT, nobody ever calls PreserveWrapper. Presumably, the CC machinery should
take care of us here.
Bug 998377 - Remove mDelayedDisconnect and friends. r=smaug