Add 'mint_oauth_token' method to RPC client.

appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py

Index: appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py
diff --git a/appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py b/appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py
index d12cffcf20095b31e02e0cfe0c0106d3381749b7..8990ef17e7af4532c3043e043b71938a57211804 100644
--- a/appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py
+++ b/appengine/swarming/swarming_bot/bot_code/remote_client_grpc.py
@@ -14,6 +14,7 @@ import grpc
 import google.protobuf.json_format
 from proto_bot import swarming_bot_pb2
 from remote_client_errors import InternalError
+from remote_client_errors import MintOAuthTokenError
 from remote_client_errors import PollError
 from utils import net
 
@@ -195,6 +196,11 @@ class RemoteClientGrpc(object):
   def ping(self):
     pass
 
+  def mint_oauth_token(self, task_id, bot_id, account_id, scopes):
+    # pylint: disable=unused-argument
+    raise MintOAuthTokenError(
+        'mint_oauth_token is not supported in grpc protocol')
+
 
 def create_state_proto(state_dict, message):
   """ Constructs a State message out of a state dict.