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

chrome/browser/resource_coordinator/tab_manager.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/resource_coordinator/tab_manager.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <set>
@@ skipping 811 matching lines @@
   GetWebContentsData(new_contents)->SetDiscardState(false);
   // When ActiveTabChanged, |new_contents| purged state changes to be false.
   GetWebContentsData(new_contents)->set_is_purged(false);
   // If |old_contents| is set, that tab has switched from being active to
   // inactive, so record the time of that transition.
   if (old_contents) {
     GetWebContentsData(old_contents)->SetLastInactiveTime(NowTicks());
     // 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);
   }
 }
 
 void TabManager::TabInsertedAt(TabStripModel* tab_strip_model,
                                content::WebContents* contents,
                                int index,
                                bool foreground) {
   // Only interested in background tabs, as foreground tabs get taken care of by
   // ActiveTabChanged.
   if (foreground)
@@ skipping 94 matching lines @@
     browser_info.window_is_active = browser->window()->IsActive();
     browser_info.window_is_minimized = browser->window()->IsMinimized();
     browser_info.window_bounds = browser->window()->GetBounds();
     browser_info.browser_is_app = browser->is_app();
     browser_info_list.push_back(browser_info);
   }
 
   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