Reviewed by Gustavo Noronha Silva.

        Bug 41340 - [GStreamer] Subtle race condition during seeks
        https://bugs.webkit.org/show_bug.cgi?id=41340

        * platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp:
        (webKitWebSrcStop):
        (webKitWebSrcNeedDataMainCb):
        (webKitWebSrcNeedDataCb):
        (webKitWebSrcEnoughDataMainCb):
        (webKitWebSrcEnoughDataCb):
        (webKitWebSrcSeekDataCb):
        (StreamingClient::didFinishLoading):
        Fix two subtle race conditions that can happen during seeks:
        - The timeout callback could be called before the callback ID is
          assigned to the instance private data. This causes the ID to
          be set after the callback has finished and breaks all future
          processing.
        - The source must not go EOS while a seek is pending because
          this confuses appsrc due to the different threads involved here.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@62955 268f45cc-cd09-0410-ab3c-d52691b4dbfc