gerrit_util: Fix NameError in GceAuthenticator._get

Bug: None
Change-Id: I6a6a210069da0f77ebbed4c6ef530864cb2d8109
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5644850
Reviewed-by: Gavin Mak <gavinmak@google.com>
Auto-Submit: Michael Cui <mlcui@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

gerrit_util.py

@@ -665,7 +665,7 @@ class GceAuthenticator(Authenticator):
             p = urllib.parse.urlparse(url)
             if p.scheme not in ('http', 'https'):
                 raise RuntimeError("Don't know how to work with protocol '%s'" %
-                                   protocol)
+                                   p.scheme)
             try:
                 resp, contents = httplib2.Http().request(url, 'GET', **kwargs)
             except (socket.error, httplib2.HttpLib2Error,