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

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 122f6bd73eb70bc170bab67c512cbc4fa47f4678..53e26cb45ae48231b701d5fa67bd2fa825eefc52 100644
--- a/chrome/browser/resource_coordinator/tab_manager.cc
+++ b/chrome/browser/resource_coordinator/tab_manager.cc
@@ -829,6 +829,9 @@ void TabManager::ActiveTabChanged(content::WebContents* old_contents,
     // Re-setting time-to-purge every time a tab becomes inactive.
     GetWebContentsData(old_contents)
         ->set_time_to_purge(GetTimeToPurge(min_time_to_purge_));
+    // If |old_contents| isn't set, we didn't switch from anything, so don't
+    // record it.
+    RecordSwitchToTab(new_contents);
   }
 }
 
@@ -943,4 +946,16 @@ std::vector<TabManager::BrowserInfo> TabManager::GetBrowserInfoList() const {
   return browser_info_list;
 }
 
+void TabManager::RecordSwitchToTab(content::WebContents* contents) const {
+  // TODO(shaseley): Change this test when the TabManager knows if we're in a
+  // background tab loading state. In that case, we'll want a separate metric
+  // for that case, and no metrics reported if we are simulaneously in session
+  // restore and background tab loading.

[fmeawad, 2017/06/27 21:22:02]

nit: This TODO is implicit from the TODOs in the header file.

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

OK, that makes sense, I'll remove this comment.

+  if (is_session_restore_loading_tabs_) {
+    UMA_HISTOGRAM_ENUMERATION("TabManager.SessionRestore.SwitchToTab",
+                              GetWebContentsData(contents)->tab_loading_state(),
+                              TAB_LOADING_STATE_MAX);
+  }
+}
+
 }  // namespace resource_coordinator