WKBasedNavigationManagerImpl::CommitPendingItem() has a special case for empty window open and DCHECKS that |webView.URL| is an about:blank URL in this case.

Due to NSURL bug radar/44566369, NSURL is not correctly parsing URL fragment from an about:blank URL. It is possible to get into this code path with |webView.URL| equal to "about:blank%23fragment", where "%23" is the percent-encoded "#". So this DCHECK has to be implemented as a string prefix.

We should change the DCHECK to using GURL::IsAboutBlank() once the NSURL bug is fixed.