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

chrome/browser/resource_coordinator/tab_manager.h

Index: chrome/browser/resource_coordinator/tab_manager.h
diff --git a/chrome/browser/resource_coordinator/tab_manager.h b/chrome/browser/resource_coordinator/tab_manager.h
index 3cd08c9805cdf1b94d247b258eccbf52a03d346f..90e9f4d8446b5b92477ac9f0a3cd90fe9c4bd6a9 100644
--- a/chrome/browser/resource_coordinator/tab_manager.h
+++ b/chrome/browser/resource_coordinator/tab_manager.h
@@ -156,6 +156,11 @@ class TabManager : public TabStripModelObserver {
   // the WebContents could be deleted if the user closed the tab.
   static int64_t IdFromWebContents(content::WebContents* web_contents);
 
+  void OnSessionRestoreStarted();
+  void OnSessionRestoreEnded();
+
+  bool IsInSessionRestore() const { return in_session_restore_; }
+
  private:
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, PurgeBackgroundRenderer);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ActivateTabResetPurgeState);
@@ -383,6 +388,8 @@ class TabManager : public TabStripModelObserver {
   // List of observers that will receive notifications on state changes.
   base::ObserverList<TabManagerObserver> observers_;
 
+  bool in_session_restore_;
+
   // Weak pointer factory used for posting delayed tasks.
   base::WeakPtrFactory<TabManager> weak_ptr_factory_;