[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..f5a66ba96ca7c0dd4a5dd21515f3a0d3a97b8b2d 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.

[zhenw, 2017/06/29 15:44:00]

nit: The comment here is describing the case where the if-check fails. But the
situation here is when |old_contents| is set. So maybe tweak it a little.
Something like: Only record when switch happens, i.e., when |old_contents| is
set.

Or you do not even need this comment, because there is comment above saying a
tab has switched when |old_contents| is set.

Either is fine to me.

[shaseley, 2017/06/29 16:20:50]

Done, I changed the comment.

+    RecordSwitchToTab(new_contents);
   }
 }
 
@@ -943,4 +946,12 @@ std::vector<TabManager::BrowserInfo> TabManager::GetBrowserInfoList() const {
   return browser_info_list;
 }
 
+void TabManager::RecordSwitchToTab(content::WebContents* contents) const {
+  if (is_session_restore_loading_tabs_) {
+    UMA_HISTOGRAM_ENUMERATION("TabManager.SessionRestore.SwitchToTab",
+                              GetWebContentsData(contents)->tab_loading_state(),
+                              TAB_LOADING_STATE_MAX);
+  }
+}
+
 }  // namespace resource_coordinator