Rework iOS recipes to use latest MB approach

scripts/slave/recipes/ios/try.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 = [
   'gclient',
   'ios',
   'json',
   'platform',
   'properties',
@@ skipping 180 matching lines @@
       buildername='ios',
       buildnumber='0',
       issue=123456,
       mastername='tryserver.fake',
       patchset=1,
       rietveld='fake://rietveld.url',
       slavename='fake-vm',
     )
     + api.ios.make_test_build_config({
       'xcode version': 'fake xcode version',
-      'GYP_DEFINES': {},
+      'GYP_DEFINES': [],
+      'gn_args': [
+        'target_os="ios"',
+        'ios_enable_code_signing=false',
+      ],
+      'mb_type': 'gn',
       'compiler': 'ninja',
       'configuration': 'Debug',
       'sdk': 'iphonesimulator8.0',
       'tests': [
         {
           'app': 'fake tests',
           'device type': 'fake device',
           'os': '8.1',
         },
       ],
-      'use_mb': True,
     })
   )