Reviewed by Martin Robinson.

        [GTK] Do not destroy WorkQueue event sources unless they have been cancelled
        https://bugs.webkit.org/show_bug.cgi?id=57611

        * Platform/WorkQueue.h:
        * Platform/gtk/WorkQueueGtk.cpp:
        (WorkQueue::EventSource::EventSource): Remove unused member
        m_dispatchSource.
        (WorkQueue::EventSource::executeEventSource): Make it return void
        instead of boolean since we are always ignoring the return value.
        (WorkQueue::EventSource::performWork): Return FALSE from the
        callback only when the source has been cancelled (condition = 0)
        to make sure it's destroyed when the even source handler is
        unregistered.
        (WorkQueue::registerEventSourceHandler): Use GRefPtr for the
        source, to avoid leaking it.
        (WorkQueue::scheduleWorkOnSource): Receive the source callback as
        parameter so that it can be used by scheduleWorkOnTermination() too.
        (WorkQueue::scheduleWork):
        (WorkQueue::scheduleWorkAfterDelay):
        (WorkQueue::scheduleWorkOnTermination): Use
        scheduleWorkOnSource().

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83281 268f45cc-cd09-0410-ab3c-d52691b4dbfc