Use NavigatorContentUtils guard for unregisterProtocolHandler()

As unregisterProtocolHandler() is shipped, it would be good to use NavigatorContentUtils REF for it.

BUG=392016

Review URL: https://codereview.chromium.org/394463004

git-svn-id: svn://svn.chromium.org/blink/trunk@178241 bbb929c8-8fbe-4397-9dbb-9b2b20218538