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        Reviewed by Tor Arne Vestbø.

        Implement a locking and scheduling mechanism for http testing sessions to be able
        to run multiple instances of run-webkit-tests parallel on the same machine.
        If a test session wants to run http tests and this feature is enabled, the pending
        sessions create lockfiles with sequential lock numbers. These locks are used to schedule
        the running test sessions in first come first served order. An exclusive lock ensures
        that the lock numbers are sequential to avoid deadlocks and starvation.
        Because the buildbot master specifies the flags used by slaves we need an environment
        variable too to be able to use the feature per-slave.
        Exporting WEBKIT_WAIT_FOR_HTTPD=1 before testing or using the --wait-for-httpd
        flag enables this feature, otherwise this patch has no effect on the testing whatsoever.

        https://bugs.webkit.org/show_bug.cgi?id=33153



git-svn-id: svn://svn.chromium.org/blink/trunk@54342 bbb929c8-8fbe-4397-9dbb-9b2b20218538