[Tab Metrics] Added TabManager.SessionRestore.SwitchToTab UMA.

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 414b743c8426e5c474210a5a3600db6db3841f95..5602203849111b6ec85ac87ac826eb1d9c8dbcf3 100644
--- a/chrome/browser/resource_coordinator/tab_manager.h
+++ b/chrome/browser/resource_coordinator/tab_manager.h
@@ -184,6 +184,7 @@ class TabManager : public TabStripModelObserver {
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, FastShutdownSingleTabProcess);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest,
                            GetUnsortedTabStatsIsInVisibleWindow);
+  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, HistogramsSessionRestoreSwitchToTab);
 
   // Information about a Browser.
   struct BrowserInfo {
@@ -330,6 +331,13 @@ class TabManager : public TabStripModelObserver {
   void OnSessionRestoreStartedLoadingTabs();
   void OnSessionRestoreFinishedLoadingTabs();
 
+  // Records UMA histograms for the tab state when switching to a different tab
+  // during session restore and background tab loading.
+  //
+  // TODO(shaseley): Add background tab loading stats when it gets integrated

[lpy, 2017/06/27 22:30:36]

Merge this TODO into the one in .cc, it doesn't give any information along with
the API.

[shaseley, 2017/06/27 23:22:10]

Per fmeawad@'s comment, I was planning to remove the one in the .cc. Is it OK to
just remove both since the code is consistent as is?

+  // into the TabManager.
+  void RecordSwitchToTab(content::WebContents* contents) const;
+
   // Timer to periodically update the stats of the renderers.
   base::RepeatingTimer update_timer_;