Record the number of tabs in a session restore in SampledProfile

chrome/browser/sessions/session_restore.h

Index: chrome/browser/sessions/session_restore.h
diff --git a/chrome/browser/sessions/session_restore.h b/chrome/browser/sessions/session_restore.h
index f756f10a0e6ae9940f161e5a749bd4630f71a451..4a582978cbe84bd384748a55931e17f8a5ce6ecd 100644
--- a/chrome/browser/sessions/session_restore.h
+++ b/chrome/browser/sessions/session_restore.h
@@ -40,12 +40,12 @@ class SessionRestore {
   };
 
   // Notification callback list.
-  using CallbackList = base::CallbackList<void(void)>;
+  using CallbackList = base::CallbackList<void(int)>;
 
   // Used by objects calling RegisterOnSessionRestoredCallback() to de-register
   // themselves when they are destroyed.
   using CallbackSubscription =
-      scoped_ptr<base::CallbackList<void(void)>::Subscription>;
+      scoped_ptr<base::CallbackList<void(int)>::Subscription>;
 
   // Restores the last session. |behavior| is a bitmask of Behaviors, see it
   // for details. If |browser| is non-null the tabs for the first window are
@@ -92,9 +92,10 @@ class SessionRestore {
   static bool IsRestoringSynchronously();
 
   // Register callbacks for session restore events. These callbacks are stored
-  // in on_session_restored_callbacks_.
+  // in |on_session_restored_callbacks_|. The "(int)" arg is the number of tabs
+  // being restored.

[sky, 2015/02/05 17:05:19]

This isn't quite right. You should clearly document the two possible values so
that two years from now when someone comes to this code and tries to add
something they understand what it really is.

[Simon Que, 2015/02/05 19:00:24]

I thought I'm passing in the number of tabs in both places where the callback
notification is invoked.

Are you referring to the two different types of notifications here?

[sky, 2015/02/06 00:55:00]

That is some what true, but there is a big difference between
synchronous/non-synchronous that should be documented.
Here's a shot at what I think we want:
The callback is supplied a tab count. The exact meaning, and timing, depends
upon the restore type:
. SessionRestore::SYNCHRONOUS: the parameter is the number of tabs that were
created. Additionally the callback is invoked immediately after the tabs have
been created. That is, the tabs are not necessarily loading.
. For all other restore types the parameter is the number of tabs that were
restored and is sent after all tabs have started loading. Additionally if a
request to restore tabs comes in while a previous request to restore tabs has
not yet completed (loading tabs is throttled), then the callback is only
notified once both sets of tabs have started loading and with the total number
of tabs for both restores.

[Simon Que, 2015/02/11 00:05:28]

Can we talk about this over video hangout tomorrow? I'll set up a half hour slot
after lunch.

   static CallbackSubscription RegisterOnSessionRestoredCallback(
-      const base::Closure& callback);
+      const base::Callback<void(int)>& callback);
 
   // The max number of non-selected tabs SessionRestore loads when restoring
   // a session. A value of 0 indicates all tabs are loaded at once.