Test: fast/events/message-channel-gc-3.html

        https://bugs.webkit.org/show_bug.cgi?id=21769
        MessagePort should be GC protected if there are messages to be delivered

        * dom/MessagePort.h: Removed pending activity count. Now we track if a close event is
        pending, and check if the queue is non-empty.
        (WebCore::MessagePort::workerContext): Added a stub implementation for a cross-heap GC bug
        fix (below).

        * dom/MessagePort.cpp:
        (WebCore::CloseMessagePortTimer::fired):
        (WebCore::MessagePort::MessagePort):
        (WebCore::MessagePort::queueCloseEvent):
        (WebCore::MessagePort::dispatchCloseEvent):
        (WebCore::MessagePort::hasPendingActivity):
        Track message and close event activity separately.

        * bindings/js/JSDOMBinding.cpp:
        (WebCore::markCrossHeapDependentObjectsForDocument): Fixed a bug in cross-heap GC that was
        causing same-heap ports to never be deleted.

        * wtf/MessageQueue.h:
        (WTF::::isEmpty): Added. Also added a warning for methods that return a snapshot of queue
        state, thus likely to cause race conditions.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37771 268f45cc-cd09-0410-ab3c-d52691b4dbfc