Reviewed by Anders Carlsson.

        WebKit2 flashes if entering and exiting compositing rapidly
        https://bugs.webkit.org/show_bug.cgi?id=60198

        If a page enters then immediately leaves compositing mode, the
        DrawingAreaImpl would send a message to the UI process that it
        had entered compositing even after all the layers had been torn
        down. Fix this by never sending the 'enter' message if we're
        waiting for the 'exit' timer to fire.

        Also fix an assertion in the UI process caused by sending mismatched
        'enter' and 'leave' messages by keeping track of which messages
        we've sent.

        * WebProcess/WebPage/DrawingAreaImpl.cpp:
        (WebKit::DrawingAreaImpl::DrawingAreaImpl):
        (WebKit::DrawingAreaImpl::layerHostDidFlushLayers):
        (WebKit::DrawingAreaImpl::exitAcceleratedCompositingMode):
        (WebKit::DrawingAreaImpl::exitAcceleratedCompositingModeSoon):
        * WebProcess/WebPage/DrawingAreaImpl.h:
        (WebKit::DrawingAreaImpl::exitAcceleratedCompositingModePending):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@85780 268f45cc-cd09-0410-ab3c-d52691b4dbfc