Reviewed by Martin Robinson.

        [GTK] Implement scheduleWorkAfterDelay() in WorkQueueGtk
        https://bugs.webkit.org/show_bug.cgi?id=57434

        * Platform/WorkQueue.h:
        * Platform/gtk/WorkQueueGtk.cpp:
        (WorkQueue::EventSource::executeEventSource): This new method
        contains the common code to execute a work item.
        (WorkQueue::EventSource::performWorkOnce): Use
        executeEventSource() to execute the work item.
        (WorkQueue::EventSource::performWork): Use executeEventSource() to
        execute the work item.
        (WorkQueue::registerEventSourceHandler): Use a GSocket instead of
        a GIOChannel since the API is newer and allows us to pass a
        cancellable object to be able to cancel the source.
        (WorkQueue::scheduleWorkOnSource): This new method contains the
        common code to attach a source to a context. It doesn't use a lock
        anymore, since g_source_attach() uses its own mutex internally.
        (WorkQueue::scheduleWork): Use an idle source instead of a timeout
        one, changing the priority to G_PRIORITY_DEFAULT.
        (WorkQueue::scheduleWorkAfterDelay): Implement it using a timeout
        source with the given delay.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83278 268f45cc-cd09-0410-ab3c-d52691b4dbfc