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 2726 matching lines @@
       "memory/tab_manager_delegate_chromeos.cc",
       "memory/tab_manager_delegate_chromeos.h",
       "memory/tab_manager_observer.cc",
       "memory/tab_manager_observer.h",
       "memory/tab_manager_web_contents_data.cc",
       "memory/tab_manager_web_contents_data.h",
       "memory/tab_stats.cc",
       "memory/tab_stats.h",
       "metrics/first_web_contents_profiler.cc",
       "metrics/first_web_contents_profiler.h",
+      "metrics/tab_reactivation_tracker.cc",
+      "metrics/tab_reactivation_tracker.h",
+      "metrics/tab_usage_recorder.cc",
+      "metrics/tab_usage_recorder.h",
       "net/firefox_proxy_settings.cc",
       "net/firefox_proxy_settings.h",
       "net/utility_process_mojo_proxy_resolver_factory.cc",
       "net/utility_process_mojo_proxy_resolver_factory.h",
       "obsolete_system/obsolete_system.h",
       "obsolete_system/obsolete_system_linux.cc",
       "obsolete_system/obsolete_system_mac.cc",
       "obsolete_system/obsolete_system_win.cc",
       "pdf/pdf_extension_util.cc",
       "pdf/pdf_extension_util.h",
@@ skipping 1485 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",
     ]
   }
 }