Reviewed by Maciej.

    Reviewed by Maciej.

        - fix http://bugzilla.opendarwin.org/show_bug.cgi?id=7318
          REGRESSION: Timer bug causes gif animation and (html and JS) page forwarding to fail

        Timers were stopping firing after a while.

        My testing makes this look like a CFRunLoopTimer bug, and it would be nice to reproduce
        this with a standalone test and report it to the CF team, but in the mean time, change
        to create a new timer each time instead of reusing the existing timer.

        * platform/mac/SharedTimerMac.cpp:
        (WebCore::setSharedTimerFiredFunction): Added an assertion.
        (WebCore::setSharedTimerFireTime): Destroy the existing timer and create a new one every time.
        Also create a non-repeating timer rather than a repeating timer.
        (WebCore::stopSharedTimer): Destroy the existing timer rather than setting its fire date
        far into the future.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@12901 268f45cc-cd09-0410-ab3c-d52691b4dbfc