Reviewed by John Sullivan.

        Flashing as hardware layers are created and destroyed in some content
        <rdar://problem/7032246>

        There's a window of time between the end of one runloop cycle, after CA layers changes
        have been committed, and the window display at the start of the next cycle when
        CA may push updates to the screen before AppKit has drawn the view contents. If
        we know that we need to do drawing synchronization (which we do when content moves
        between a layer and the view), then we need to call -disableScreenUpdatesUntilFlush
        from the existing runloop observer that is set up when layers need repainting to
        ensure that layer updates don't reach the screen before view-based painting does.

        * WebView/WebHTMLView.mm:
        (-[WebHTMLView drawRect:]):
        Tweak the comments

        * WebView/WebView.mm:
        (viewUpdateRunLoopObserverCallBack):
        (-[WebView _scheduleViewUpdate]):
        Add a big comment to explain the timing of things.
        Call -disableScreenUpdatesUntilFlush when the view _needsOneShotDrawingSynchronization.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@45745 268f45cc-cd09-0410-ab3c-d52691b4dbfc