Reviewed by Dan Bernstein.

        Add PageClientImpl::containingWindowGraphicsContext
        https://bugs.webkit.org/show_bug.cgi?id=52666

        * UIProcess/API/mac/PageClientImpl.h:
        * UIProcess/API/mac/PageClientImpl.mm:
        (WebKit::PageClientImpl::containingWindowGraphicsContext):
        * UIProcess/PageClient.h:
        * UIProcess/WebPageProxy.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76069 268f45cc-cd09-0410-ab3c-d52691b4dbfc