GCAPI should append to the existing experiment_labels instead of clobbering them.

chrome/chrome_tests_unit.gypi

 # Copyright 2012 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.
 {
   'targets': [
     {
       # This target contains mocks and test utilities that don't belong in
       # production libraries but are used by more than one test executable.
       'target_name': 'test_support_common',
       'type': 'static_library',
@@ skipping 1825 matching lines @@
         'common/importer/firefox_importer_utils_unittest.cc',
         'common/instant_restricted_id_cache_unittest.cc',
         'common/mac/cfbundle_blocker_unittest.mm',
         'common/mac/mock_launchd.cc',
         'common/mac/mock_launchd.h',
         'common/mac/objc_method_swizzle_unittest.mm',
         'common/mac/objc_zombie_unittest.mm',
         'common/metrics/caching_permuted_entropy_provider_unittest.cc',
         'common/metrics/metrics_log_base_unittest.cc',
         'common/metrics/metrics_log_manager_unittest.cc',
+        'common/metrics/variations/experiment_labels_win_unittest.cc',
         'common/metrics/variations/variations_util_unittest.cc',
         'common/multi_process_lock_unittest.cc',
         'common/net/url_fixer_upper_unittest.cc',
         'common/net/x509_certificate_model_unittest.cc',
         'common/partial_circular_buffer_unittest.cc',
         'common/pref_names_util_unittest.cc',
         'common/search_urls_unittest.cc',
         'common/service_process_util_unittest.cc',
         'common/switch_utils_unittest.cc',
         'common/thumbnail_score_unittest.cc',
@@ skipping 1053 matching lines @@
             'browser/ui/app_list/test/fake_profile.h',
             'browser/ui/app_list/test/fake_profile_store.cc',
             'browser/ui/app_list/test/fake_profile_store.h',
             'browser/ui/app_list/test/fast_show_pickler_unittest.cc',
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }