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

chrome/test/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/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
@@ skipping 4378 matching lines @@
 
   if (enable_captive_portal_detection) {
     sources += [
       "../browser/captive_portal/captive_portal_service_unittest.cc",
       "../browser/captive_portal/captive_portal_tab_helper_unittest.cc",
       "../browser/captive_portal/captive_portal_tab_reloader_unittest.cc",
     ]
   }
   if (enable_session_service) {
     sources += [
+      "../browser/sessions/session_restore_observer_unittest.cc",
       "../browser/sessions/session_restore_stats_collector_unittest.cc",
       "../browser/sessions/session_service_unittest.cc",
       "../browser/sessions/tab_loader_unittest.cc",
     ]
 
     if (is_mac && !mac_views_browser) {
       sources += [ "../browser/ui/startup/session_crashed_infobar_delegate_unittest.cc" ]
     }
   }
   if (enable_webrtc) {
@@ skipping 757 matching lines @@
 }
 
 if (is_win) {
   loadable_module("conflicts_dll") {
     testonly = true
     sources = [
       "conflicts/conflicts_dll.cc",
     ]
   }
 }