swarming: add transaction_id to tasks.new request

appengine/swarming/swarming_rpcs.py

Index: appengine/swarming/swarming_rpcs.py
diff --git a/appengine/swarming/swarming_rpcs.py b/appengine/swarming/swarming_rpcs.py
index 980d133c7704edf0eebe6b60e493e1b00b24d338..b7eed2043125fc6668347db578103092b15e5144 100644
--- a/appengine/swarming/swarming_rpcs.py
+++ b/appengine/swarming/swarming_rpcs.py
@@ -236,6 +236,15 @@ class NewTaskRequest(messages.Message):
   # Will be but into "userdata" fields of PubSub message.
   pubsub_userdata = messages.StringField(11)
 
+  # Global ID of the transaction to create a new task request.
+  # If specified and a task request with the same transaction id was created
+  # recently, the existing request will be returned as is.
+  # If two NewTaskRequests use the same transaction ID, requests must have all
+  # attributes equal.
+  #
+  # Transaction ID can be used to cancel this task request.
+  transaction_id = messages.StringField(12)
+
 
 class TaskRequest(messages.Message):
   """Description of a task request as registered by the server."""