Make webrtc recipes explicitly set GIT_MODE

scripts/slave/recipes/webrtc/auto_roll_webrtc_deps.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 
 DEPS = [
   'depot_tools/bot_update',
   'depot_tools/gclient',
   'depot_tools/git',
   'recipe_engine/path',
   'recipe_engine/properties',
   'recipe_engine/python',
   'recipe_engine/raw_io',
   'webrtc',
 ]
 
 
 def RunSteps(api):
-  api.gclient.set_config('webrtc')
+  api.gclient.set_config('webrtc', GIT_MODE=True)
 
   step_result = api.python(
         'check roll status',
         api.path['build'].join('scripts', 'tools', 'pycurl.py'),
         args=['https://webrtc-roll-cr-rev-status.appspot.com/status'],
         stdout=api.raw_io.output(),
         step_test_data=lambda: api.raw_io.test_api.stream_output(
             '1', stream='stdout')
   )
   step_result.presentation.logs['stdout'] = step_result.stdout.splitlines()
@@ skipping 32 matching lines @@
       api.test('rolling_activated') +
       api.properties.generic(mastername='client.webrtc.fyi',
                              buildername='Auto-roll - WebRTC DEPS')
   )
   yield (api.test('rolling_deactivated') +
       api.properties.generic(mastername='client.webrtc.fyi',
                              buildername='Auto-roll - WebRTC DEPS') +
       api.override_step_data('check roll status',
                              api.raw_io.stream_output('0', stream='stdout'))
   )