Using bot_id from tasks/list query response instead of parsing for id

From what I can see, bot_id is a gauranteed field in the query response
vs assuming it comes back as a tag.

Bug: 831252
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I6cb0c48a1dce8ffcea35ce3ab65404366c450c9a
Reviewed-on: https://chromium-review.googlesource.com/1047065
Commit-Queue: Emily Hanley <eyaich@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556440}

testing/trigger_scripts/perf_device_trigger.py

@@ -222,9 +222,11 @@ class PerfDeviceTriggerer(base_test_triggerer.BaseTestTriggerer):
       # We queried with a limit of 1 so we could only get back
       # the most recent which is what we care about.
       task = tasks[0]
+      if 'bot_id' in task:
+        return task['bot_id']
       for tag in task['tags']:
-        if 'id' in tag:
-          return tag[len('id;'):]
+        if tag.startswith('id:'):
+          return tag[len('id:'):]
     # No eligible shard for this bot
     return None
 

testing/trigger_scripts/perf_device_trigger_unittest.py

@@ -99,7 +99,7 @@ class UnitTest(unittest.TestCase):
     for i in xrange(num_shards):
       bot_id = previous_task_assignment_map.get(i)
       files['base_trigger_dimensions%d.json' % file_index] = (
-          self.generate_last_task_to_shard_query_response(bot_id))
+          self.generate_last_task_to_shard_query_response(i, bot_id))
       file_index = file_index + 1
     for i in xrange(num_shards):
       task = {
@@ -120,9 +120,14 @@ class UnitTest(unittest.TestCase):
       file_index = file_index + 1
     return files
 
-  def generate_last_task_to_shard_query_response(self, bot_id):
+  def generate_last_task_to_shard_query_response(self, shard, bot_id):
     if len(bot_id):
-      return {'items': [{'tags': [('id:%s' % bot_id)]}]}
+      # Test both cases where bot_id is present and you have to parse
+      # out of the tags.
+      if shard % 2:
+        return {'items': [{'bot_id': bot_id}]}
+      else:
+        return {'items': [{'tags': [('id:%s' % bot_id)]}]}
     return {}
 
   def generate_list_of_eligible_bots_query_response(