Reviewed by Sam Weinig.

        Add an asynchronous WKPageForceRepaint
        https://bugs.webkit.org/show_bug.cgi?id=52964
        <rdar://problem/8898527>

        * UIProcess/API/C/WKPage.cpp:
        (WKPageForceRepaint):
        Call WebPageProxy::forceRepaint.

        * UIProcess/API/C/WKPage.h:
        Add WKPageForceRepaint.

        * UIProcess/GenericCallback.h:
        Add a "generic" VoidCallback class.

        * UIProcess/WebPageProxy.cpp:
        (WebKit::WebPageProxy::forceRepaint):
        Insert the callback in the m_voidCallbacks map and send a forceRepaint message.

        (WebKit::WebPageProxy::voidCallback):
        Call the right void callback.

        (WebKit::WebPageProxy::processDidCrash):
        Invalidate m_voidCallbacks.

        * UIProcess/WebPageProxy.messages.in:
        Add a VoidCallback message.

        * WebProcess/WebPage/ChunkedUpdateDrawingArea.cpp:
        (WebKit::ChunkedUpdateDrawingArea::forceRepaint):
        Force a repaint.

        * WebProcess/WebPage/ChunkedUpdateDrawingArea.h:
        Add forceRepaint.

        * WebProcess/WebPage/WebPage.cpp:
        (WebKit::WebPage::forceRepaint):
        Call forceRepaint on the drawing area.

        * WebProcess/WebPage/WebPage.messages.in:
        Add a ForceRepaint message.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76452 268f45cc-cd09-0410-ab3c-d52691b4dbfc