[TabMetrics] Add signals that mark the start and end of session restore.

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 2702 matching lines @@
       "ssl/captive_portal_metrics_recorder.cc",
       "ssl/captive_portal_metrics_recorder.h",
     ]
   }
   if (enable_session_service) {
     sources += [
       "sessions/session_restore.cc",
       "sessions/session_restore.h",
       "sessions/session_restore_delegate.cc",
       "sessions/session_restore_delegate.h",
+      "sessions/session_restore_observer.cc",
+      "sessions/session_restore_observer.h",
       "sessions/session_restore_stats_collector.cc",
       "sessions/session_restore_stats_collector.h",
       "sessions/session_service.cc",
       "sessions/session_service.h",
       "sessions/session_service_factory.cc",
       "sessions/session_service_factory.h",
       "sessions/session_service_utils.cc",
       "sessions/session_service_utils.h",
       "sessions/tab_loader.cc",
       "sessions/tab_loader.h",
@@ skipping 2080 matching lines @@
       "//components/search_engines",
       "//rlz:rlz_lib",
     ]
   }
 }
 
 service_manifest("preferences_forwarder_manifest") {
   name = "preferences_forwarder"
   source = "prefs/forwarder_manifest.json"
 }