v8's ContextSwitcher::PreemptionReceieved() may check the wrong isolate

In v8thread.cc, ContextSwitcher::PreemptionReceived() is called from the target
thread (as opposed to the ContextSwitcher thread), yet it was asserting the
wrong isolate was locked.

Some details are here, although they do not confirm the issue with PreemptionReceived:

http://code.google.com/p/v8/issues/detail?id=1427