Reviewed by John and Darin.

    Reviewed by John and Darin.

        <rdar://problem/4634290> Cannot selectively install a custom
        scroller that differs from the default Aqua frame size.

        Adds two new private methods to WebFrameView that allows
        an application to set a custom scroll view class. This is needed
        if the application wants to install a custom scroller that is wider
        than the typical scroller, because NSScrollView does the content
        rect calculations in a class method (ignoring custom scrollers.)
        The _setScrollViewClass method requires the class to be a subclass
        of WebDynamicScrollBarView, or nil can be passed to reset to the default class.
        A new scroll view of the specified class will then replace the previous
        one without the need to reload content of the frame.

        * WebView/WebFrameView.m:
        (-[WebFrameView _customScrollViewClass]):
        (-[WebFrameView _setCustomScrollViewClass:]):
        * WebView/WebFrameViewPrivate.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@15605 268f45cc-cd09-0410-ab3c-d52691b4dbfc