[Tab Metrics] Measure FP, FCP and FMP for Foreground Tab during Session Restore

chrome/browser/resource_coordinator/tab_manager.cc

Index: chrome/browser/resource_coordinator/tab_manager.cc
diff --git a/chrome/browser/resource_coordinator/tab_manager.cc b/chrome/browser/resource_coordinator/tab_manager.cc
index 208ab52cd2b9982ad6ece62ea7bd7b8a0f0bacd9..3601c9f9c2b81e0f2d4d9574c7be0881caef20bf 100644
--- a/chrome/browser/resource_coordinator/tab_manager.cc
+++ b/chrome/browser/resource_coordinator/tab_manager.cc
@@ -112,6 +112,7 @@ TabManager::TabManager()
       browser_tab_strip_tracker_(this, nullptr, nullptr),
       test_tick_clock_(nullptr),
       in_session_restore_(false),
+      initial_active_tab_changed_(false),
       weak_ptr_factory_(this) {
 #if defined(OS_CHROMEOS)
   delegate_.reset(new TabManagerDelegate(weak_ptr_factory_.GetWeakPtr()));
@@ -793,6 +794,10 @@ void TabManager::ActiveTabChanged(content::WebContents* old_contents,
     GetWebContentsData(old_contents)
         ->set_time_to_purge(GetTimeToPurge(min_time_to_purge_));
   }
+
+  if (old_contents && !initial_active_tab_changed_) {
+    initial_active_tab_changed_ = true;

[chrisha, 2017/06/21 15:00:26]

This logic seems frail. If this is specifically tied to session restore, then it
seems like we should be resetting this when OnSessionRestoreStarted is called,
so that it tracks the initial foreground tab associated with a session restore.

Note that session restore can be invoked at anytime, not just at browser
startup.

The more I think about this, the more I like sky's idea of session restore
itself exposing an observer interface, and then tracking all of this state
directly in the PageLoadMetricObserver, which would also be a
SessionRestoreObserver and a TabStripModelObserver. Why do we need TabManager as
the middle man? Other than for these metrics, TabManager doesn't explicitly care
that the initially visible tab related to a session restore has since been
changed away from...

The session restore observer should be able to share everything about an
individual session restore, including when it starts, the tabs it is restoring
(either as WebContents, or TabStripModels and tab IDs), and when it ends.

There is already something like this: SessionRestoreStatsCollector. We could be
adding these metrics directly to that class, or alternatively, abstracting out a
SessionRestoreObserver helper from that class, and having two separate clients.

[ducbui, 2017/06/27 19:41:24]

Acknowledged. I reset the flag for each session restore.

+  }
 }
 
 void TabManager::TabInsertedAt(TabStripModel* tab_strip_model,