[WK2][Win] Fix ASSERT(DeleteTimerQueueTimer...)
https://bugs.webkit.org/show_bug.cgi?id=116039

Reviewed by Benjamin Poulain.

According to the documentation, DeleteTimerQueueTimer can be expected to return
false with an ERROR_IO_PENDING error when called from the timer's callback.

* Platform/win/WorkQueueWin.cpp:
(WorkQueue::timerCallback):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150068 268f45cc-cd09-0410-ab3c-d52691b4dbfc