Add implementation in metrics.h that uses atomic pointer.

webrtc/system_wrappers/system_wrappers_tests.gyp

 # Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'includes': ['../build/common.gypi',],
   'targets': [
     {
       'target_name': 'system_wrappers_unittests',
       'type': '<(gtest_target_type)',
       'dependencies': [
         '<(DEPTH)/testing/gtest.gyp:gtest',
         '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
+        '<(webrtc_root)/test/test.gyp:histogram',
         '<(webrtc_root)/test/test.gyp:test_support_main',
       ],
       'sources': [
         'source/aligned_array_unittest.cc',
         'source/aligned_malloc_unittest.cc',
         'source/clock_unittest.cc',
         'source/condition_variable_unittest.cc',
         'source/critical_section_unittest.cc',
         'source/logging_unittest.cc',
         'source/data_log_unittest.cc',
         'source/data_log_unittest_disabled.cc',
         'source/data_log_helpers_unittest.cc',
         'source/data_log_c_helpers_unittest.c',
         'source/data_log_c_helpers_unittest.h',
+        'source/metrics_unittest.cc',
         'source/ntp_time_unittest.cc',
         'source/rtp_to_ntp_unittest.cc',
         'source/scoped_vector_unittest.cc',
         'source/stringize_macros_unittest.cc',
         'source/stl_util_unittest.cc',
       ],
       'conditions': [
         ['enable_data_logging==1', {
           'sources!': [ 'source/data_log_unittest_disabled.cc', ],
         }, {
@@ skipping 36 matching lines @@
           ],
           'sources': [
             'system_wrappers_unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }