Introduce a MetricsDataProvider interface.

components/metrics.gypi

 # Copyright 2014 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': [
     {
       'target_name': 'metrics',
       'type': 'static_library',
       'include_dirs': [
         '..',
       ],
       'dependencies': [
         '../base/base.gyp:base',
         'component_metrics_proto',
       ],
       'sources': [
+        'metrics/metrics_data_provider.h',
         'metrics/metrics_hashes.cc',
         'metrics/metrics_hashes.h',
         'metrics/metrics_log_base.cc',
         'metrics/metrics_log_base.h',
         'metrics/metrics_log_manager.cc',
         'metrics/metrics_log_manager.h',
         'metrics/metrics_pref_names.cc',
         'metrics/metrics_pref_names.h',
         'metrics/persisted_logs.cc',
         'metrics/persisted_logs.h',
@@ skipping 14 matching lines @@
         'metrics/proto/user_action_event.proto',
       ],
       'variables': {
         'proto_in_dir': 'metrics/proto',
         'proto_out_dir': 'components/metrics/proto',
       },
       'includes': [ '../build/protoc.gypi' ],
     },
   ],
 }