Revert of Flip Linux x64, linux_chromium_archive_rel_ng bots to MB.

scripts/slave/recipe_modules/chromium_tests/chromium.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.
 
 from . import steps
 
 RESULTS_URL = 'https://chromeperf.appspot.com'
 
 SPEC = {
   'builders': {
     'Linux x64': {
       'chromium_config': 'chromium',
       'chromium_apply_config': [
         'clobber',
         'isolation_mode_noop',
-        'mb',
         'ninja_confirm_noop',
         'no_dump_symbols',
       ],
       'gclient_config': 'chromium',
       'chromium_config_kwargs': {
         'BUILD_CONFIG': 'Release',
         'TARGET_BITS': 64,
       },
       'compile_targets': [
         'all',
       ],
       'tests': [
         steps.SizesStep(RESULTS_URL, 'chromium-rel-linux-64'),
       ],
       'test_generators': [
         steps.generate_gtest,
         steps.generate_script,
         steps.generate_isolated_script,
       ],
       'archive_build': True,
       'gs_bucket': 'chromium-browser-snapshots',
       'gs_acl': 'public-read',
       'testing': {
         'platform': 'linux',
       },
     },
   },
 }