Update chromium_mac recipe for re-imaged testers.

scripts/slave/recipes/chromium_trybot.py

 # Copyright 2013 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 = [
   'bot_update',
   'chromium',
   'chromium_tests',
   'gclient',
   'isolate',
@@ skipping 315 matching lines @@
         'chromium_config': 'chromium',
         'compile_only': False,
         'testing': {
           'platform': 'mac',
         },
       },
       'mac_chromium_rel_ng': {
         'based_on_main_waterfall': {
           'mastername': 'chromium.mac',
           'buildername': 'Mac Builder',
-          'testers': ['Mac10.7 Tests (1)'],
+          'testers': ['Mac10.8 Tests'],
         },
         'testing': {
           'platform': 'mac',
         },
       },
       'mac_chromium_compile_dbg': {
         'chromium_config_kwargs': {
           'BUILD_CONFIG': 'Debug',
           'TARGET_BITS': 32,
         },
@@ skipping 1191 matching lines @@
             'test': 'base_tests',
             'args': ['--gtest-filter: *NaCl*.*'],
           },
         ],
       })
     ) +
     api.override_step_data(
       'analyze',
       api.json.output({'invalid_targets': 'invalid target'}))
   )