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

chrome/browser/sessions/session_restore.h

 // Copyright (c) 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.
 
 #ifndef CHROME_BROWSER_SESSIONS_SESSION_RESTORE_H_
 #define CHROME_BROWSER_SESSIONS_SESSION_RESTORE_H_
 
 #include <stdint.h>
 
 #include <memory>
 #include <vector>
 
 #include "base/callback_list.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
+#include "chrome/browser/sessions/session_restore_observer.h"
 #include "components/history/core/browser/history_service.h"
 #include "components/sessions/core/session_types.h"
 #include "ui/base/window_open_disposition.h"
 
 class Browser;
 class Profile;
 
 namespace content {
 class WebContents;
 }
@@ skipping 74 matching lines @@
   // have not necessarily finished loading. The integer supplied to the callback
   // indicates the number of tabs that were created.
   static CallbackSubscription RegisterOnSessionRestoredCallback(
       const base::Callback<void(int)>& callback);
 
   // Add |urls| to URLs-to-open list, so that they will be opened after session
   // URLs when session is restored.
   static void AddURLsToOpen(const Profile* profile,
                             const std::vector<GURL>& urls);
 
+  // Add/remove an observer to/from this session restore.
+  static void AddObserver(SessionRestoreObserver* observer);
+  static void RemoveObserver(SessionRestoreObserver* observer);
+
+  // Accessor for the observer list. Create the list the first time to always
+  // return a valid reference.
+  static base::ObserverList<SessionRestoreObserver>& observers() {
+    if (!observers_)
+      observers_ = new base::ObserverList<SessionRestoreObserver>();
+    return *observers_;
+  }
+
  private:
   SessionRestore();
 
   // Accessor for |*on_session_restored_callbacks_|. Creates a new object the
   // first time so that it always returns a valid object.
   static CallbackList* on_session_restored_callbacks() {
     if (!on_session_restored_callbacks_)
       on_session_restored_callbacks_ = new CallbackList();
     return on_session_restored_callbacks_;
   }
 
   // Contains all registered callbacks for session restore notifications.
   static CallbackList* on_session_restored_callbacks_;
 
+  // Contains all registered observers for session restore events.
+  static base::ObserverList<SessionRestoreObserver>* observers_;

[chrisha, 2017/06/27 17:09:07]

Shouldn't this explicitly use a leaky lazy instance, or some other annotation
that explains that this leaks? Otherwise we could end up with LSAN errors...

+
   DISALLOW_COPY_AND_ASSIGN(SessionRestore);
 };
 
 #endif  // CHROME_BROWSER_SESSIONS_SESSION_RESTORE_H_