Add a default implementation in metrics_default.cc of histograms methods in system_wrappers/interfac

webrtc/system_wrappers/system_wrappers.gyp

 # Copyright (c) 2012 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', ],
@@ skipping 162 matching lines @@
       'target_name': 'field_trial_default',
       'type': 'static_library',
       'sources': [
         'include/field_trial_default.h',
         'source/field_trial_default.cc',
       ]
     }, {
       'target_name': 'metrics_default',
       'type': 'static_library',
       'sources': [
+        'include/metrics_default.h',
         'source/metrics_default.cc',
       ],
     }, {
       'target_name': 'system_wrappers_default',
       'type': 'static_library',
       'dependencies': [
         'system_wrappers',
         'field_trial_default',
         'metrics_default',
       ]
     },
   ], # targets
 }