<rdar://problem/3600734> API: please add a way to turn vertical scrollbar
        always on (for Mail, to avoid reflow when typing)

        Adds new methods to lock the scrolling mode on WebDynamicScrollBarsView.
        Locking the scroll mode prevents WebCore from changing it as needed.
        Also adds an SPI on WebView that will lock the "always on" mode
        for each scroller.

        * WebKit.xcodeproj/project.pbxproj:
        * WebView/WebDynamicScrollBarsView.h:
        * WebView/WebDynamicScrollBarsView.m:
        (-[WebDynamicScrollBarsView setAllowsScrolling:]):
        (-[WebDynamicScrollBarsView allowsScrolling]):
        (-[WebDynamicScrollBarsView setAllowsHorizontalScrolling:]):
        (-[WebDynamicScrollBarsView setAllowsVerticalScrolling:]):
        (-[WebDynamicScrollBarsView setHorizontalScrollingMode:]):
        (-[WebDynamicScrollBarsView setVerticalScrollingMode:]):
        (-[WebDynamicScrollBarsView setScrollingMode:]):
        (-[WebDynamicScrollBarsView setHorizontalScrollingModeLocked:]):
        (-[WebDynamicScrollBarsView setVerticalScrollingModeLocked:]):
        (-[WebDynamicScrollBarsView setScrollingModesLocked:]):
        (-[WebDynamicScrollBarsView horizontalScrollingModeLocked]):
        (-[WebDynamicScrollBarsView verticalScrollingModeLocked]):
        * WebView/WebView.m:
        (-[WebView setAlwaysShowVerticalScroller:]):
        (-[WebView alwaysShowVerticalScroller]):
        (-[WebView setAlwaysShowHorizontalScroller:]):
        (-[WebView alwaysShowHorizontalScroller]):
        * WebView/WebViewPrivate.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@14771 268f45cc-cd09-0410-ab3c-d52691b4dbfc