Reviewed by Darin Adler.

        Use a CGLayer for the backing store when possible
        https://bugs.webkit.org/show_bug.cgi?id=52679

        * UIProcess/mac/BackingStoreMac.mm:
        (WebKit::BackingStore::paint):
        If there is a layer, paint it into the given context.

        (WebKit::BackingStore::backingStoreContext):
        If we can get the containing window graphics context, use it to create a 
        CGLayer that we'll use for the backing store.

        (WebKit::BackingStore::scroll):
        Paint the layer into itself.

        * UIProcess/mac/WebPageProxyMac.mm:
        (WebKit::WebPageProxy::containingWindowGraphicsContext):
        Call the page client.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76080 268f45cc-cd09-0410-ab3c-d52691b4dbfc