Add metrics to keep track of the tab activate/deactivate cycle

chrome/browser/BUILD.gn

 # 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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 561 matching lines @@
     "metrics/metrics_memory_details.cc",
     "metrics/metrics_memory_details.h",
     "metrics/metrics_reporting_state.cc",
     "metrics/metrics_reporting_state.h",
     "metrics/perf/perf_provider_chromeos.cc",
     "metrics/perf/perf_provider_chromeos.h",
     "metrics/sampling_metrics_provider.cc",
     "metrics/sampling_metrics_provider.h",
     "metrics/subprocess_metrics_provider.cc",
     "metrics/subprocess_metrics_provider.h",
+    "metrics/tab_usage_recorder.cc",
+    "metrics/tab_usage_recorder.h",
     "metrics/thread_watcher.cc",
     "metrics/thread_watcher.h",
     "metrics/thread_watcher_android.cc",
     "metrics/thread_watcher_android.h",
     "metrics/thread_watcher_report_hang.cc",
     "metrics/thread_watcher_report_hang.h",
     "metrics/time_ticks_experiment_win.cc",
     "metrics/time_ticks_experiment_win.h",
     "metrics/variations/chrome_variations_service_client.cc",
     "metrics/variations/chrome_variations_service_client.h",
@@ skipping 3659 matching lines @@
   # linking all of the test support.
   static_library("pepper_cdm_test_constants") {
     testonly = true
     visibility = [ "//chrome/*" ]
     sources = [
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }