metrics: Add RandomSelector, a class that randomly selects items with odds

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.
 {
   'variables': {
     'chrome_unit_tests_sources': [
       # histograms.xml is analyzed by AboutFlagsHistogramTest, so this
       # dependency is needed to make commit bots run unit_tests on
       # histograms.xml changes.
       '../tools/metrics/histograms/histograms.xml',
@@ skipping 120 matching lines @@
       'browser/manifest/manifest_icon_selector_unittest.cc',
       'browser/media/media_stream_device_permission_context_unittest.cc',
       'browser/media/midi_permission_context_unittest.cc',
       'browser/media/native_desktop_media_list_unittest.cc',
       'browser/memory/oom_priority_manager_delegate_chromeos_unittest.cc',
       'browser/memory/oom_priority_manager_unittest.cc',
       'browser/metrics/chrome_browser_main_extra_parts_metrics_unittest.cc',
       'browser/metrics/chrome_metrics_service_accessor_unittest.cc',
       'browser/metrics/cloned_install_detector_unittest.cc',
       'browser/metrics/perf/perf_provider_chromeos_unittest.cc',
+      'browser/metrics/perf/random_selector_unittest.cc',
       'browser/metrics/signin_status_metrics_provider_chromeos_unittest.cc',
       'browser/metrics/signin_status_metrics_provider_unittest.cc',
       'browser/metrics/thread_watcher_android_unittest.cc',
       'browser/metrics/thread_watcher_unittest.cc',
       'browser/metrics/time_ticks_experiment_unittest.cc',
       'browser/metrics/variations/generated_resources_map_lookup_unittest.cc',
       'browser/mod_pagespeed/mod_pagespeed_metrics_unittest.cc',
       'browser/net/chrome_network_delegate_unittest.cc',
       'browser/net/dns_probe_runner_unittest.cc',
       'browser/net/dns_probe_service_unittest.cc',
@@ skipping 2776 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }