[GTK] Adapt WorkQueueGtk to the latest changes in WebKit2 after r141497
https://bugs.webkit.org/show_bug.cgi?id=108607

Reviewed by Anders Carlsson.

* Platform/gtk/WorkQueueGtk.cpp:
(WorkQueue::EventSource::executeEventSource): Remove the is valid
work queue check.
(WorkQueue::EventSource): WorkQueue is now refcounted, so keep a
reference when a new job is scheduled and unref it when it
finishes.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141599 268f45cc-cd09-0410-ab3c-d52691b4dbfc