Reviewed by Alexey Proskuryakov.

        flaky websocket/tests/frame-length-overflow.html
        https://bugs.webkit.org/show_bug.cgi?id=43902

        Add m_shouldDiscardReceivedData flag to indicate it will no longer
        receive data from network.

        * websockets/WebSocketChannel.cpp:
        (WebCore::WebSocketChannel::WebSocketChannel):
        (WebCore::WebSocketChannel::didReceiveData):
        (WebCore::WebSocketChannel::didFail):
        (WebCore::WebSocketChannel::processBuffer):
        - when frame length overflows, we couldn't process data any more.
          clear buffer and mark m_shouldDiscardReceivedData true to make sure
          it doesn't process the same buffer again.
        * websockets/WebSocketChannel.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@65313 268f45cc-cd09-0410-ab3c-d52691b4dbfc