Reviewed by Darin Adler.

        Factor code to create the backing store bitmap context out into a new function
        https://bugs.webkit.org/show_bug.cgi?id=52670

        * UIProcess/BackingStore.h:
        * UIProcess/mac/BackingStoreMac.mm:
        (WebKit::BackingStore::backingStoreContext):
        Create the backing store context.

        (WebKit::BackingStore::incorporateUpdate):
        Call backingStoreContext.

        (WebKit::BackingStore::scroll):
        Assert that the context is not null.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76072 268f45cc-cd09-0410-ab3c-d52691b4dbfc