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

chrome/browser/sessions/tab_loader.h

 // Copyright 2015 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_TAB_LOADER_H_
 #define CHROME_BROWSER_SESSIONS_TAB_LOADER_H_
 
 #include <stddef.h>
 
 #include <list>
@@ skipping 107 matching lines @@
   // React to memory pressure by stopping to load any more tabs.
   void OnMemoryPressure(
       base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level);
 
   // base::MemoryCoordinatorClient implementation:
   void OnMemoryStateChange(base::MemoryState state) override;
 
   // Stops loading tabs to purge memory by stopping to load any more tabs.
   void StopLoadingTabs();
 
+  // Signal the start and end of session restore to the tab manager.
+  static void MarkSessionRestoreStarted();
+  static void MarkSessionRestoreEnded();
+
   // Limit the number of loaded tabs.
   // Value of 0 restores default behavior. In test mode command line flags and
   // free memory size are not taken into account.
   static void SetMaxLoadedTabCountForTest(size_t value);
 
   std::unique_ptr<TabLoaderDelegate> delegate_;
 
   // Listens for system under memory pressure notifications and stops loading
   // of tabs when we start running out of memory.
   base::MemoryPressureListener memory_pressure_listener_;
@@ skipping 30 matching lines @@
   // explicitly referenced so that it can be notified of deferred tab loads due
   // to memory pressure.
   scoped_refptr<SessionRestoreStatsCollector> stats_collector_;
 
   static TabLoader* shared_tab_loader_;
 
   DISALLOW_COPY_AND_ASSIGN(TabLoader);
 };
 
 #endif  // CHROME_BROWSER_SESSIONS_TAB_LOADER_H_