auth: Convert GoogleMobileOAuth2Backend to accept a realm object.

zerver/views/auth.py

@@ -645,8 +645,10 @@ def api_fetch_api_key(request, username=REQ(), password=REQ()):
     # type: (HttpRequest, str, str) -> HttpResponse
     return_data = {}  # type: Dict[str, bool]
     if username == "google-oauth2-token":
+        subdomain = get_subdomain(request)
+        realm = get_realm(subdomain)
         user_profile = authenticate(google_oauth2_token=password,
-                                    realm_subdomain=get_subdomain(request),
+                                    realm=realm,
                                     return_data=return_data)
     else:
         if not ldap_auth_enabled(realm=get_realm_from_request(request)):