Add metrics regarding concurrent audible tabs in Chromium.

content/content_tests.gypi

 # Copyright (c) 2013 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.
 
 {
   'variables': {
     'layouttest_support_content_sources': [
       'public/test/layouttest_support.h',
       'public/test/nested_message_pump_android.cc',
       'public/test/nested_message_pump_android.h',
@@ skipping 485 matching lines @@
       'browser/loader/mime_type_resource_handler_unittest.cc',
       'browser/loader/navigation_url_loader_unittest.cc',
       'browser/loader/resource_buffer_unittest.cc',
       'browser/loader/resource_dispatcher_host_unittest.cc',
       'browser/loader/resource_loader_unittest.cc',
       'browser/loader/resource_scheduler_unittest.cc',
       'browser/loader/temporary_file_stream_unittest.cc',
       'browser/loader/upload_data_stream_builder_unittest.cc',
       'browser/mach_broker_mac_unittest.cc',
       'browser/media/android/media_session_uma_helper_unittest.cc',
+      'browser/media/audible_metrics_unittest.cc',
       'browser/media/audio_stream_monitor_unittest.cc',
       'browser/media/capture/audio_mirroring_manager_unittest.cc',
       'browser/media/capture/cursor_renderer_aura_unittest.cc',
       'browser/media/capture/web_contents_audio_input_stream_unittest.cc',
       'browser/media/capture/web_contents_video_capture_device_unittest.cc',
       'browser/media/media_internals_unittest.cc',
       'browser/media/midi_host_unittest.cc',
       'browser/media/webrtc_identity_store_unittest.cc',
       'browser/net/quota_policy_cookie_store_unittest.cc',
       'browser/notification_service_impl_unittest.cc',
@@ skipping 1697 matching lines @@
                   'content_unittests_apk.isolate',
                 ],
               },
             ],
           },
         ],
       ],
     }],
   ],
 }