[ObjC ARC] Converts ios/chrome/browser/metrics:metrics_internal to ARC.

ios/chrome/browser/metrics/tab_usage_recorder.mm

 // Copyright 2013 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 "ios/chrome/browser/metrics/tab_usage_recorder.h"
 
 #include "base/metrics/histogram_macros.h"
 #include "ios/chrome/browser/chrome_url_constants.h"
 #import "ios/chrome/browser/metrics/previous_session_info.h"
 #import "ios/chrome/browser/tabs/tab.h"
 #import "ios/web/web_state/ui/crw_web_controller.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 const char kTabUsageHistogramPrefix[] = "Tab";
 
 // The histogram recording the state of the tab the user switches to.
 const char kSelectedTabHistogramName[] =
     "Tab.StatusWhenSwitchedBackToForeground";
 
 // The histogram to record the number of page loads before an evicted tab is
 // selected.
 const char kPageLoadsBeforeEvictedTabSelected[] =
     "Tab.PageLoadsSinceLastSwitchToEvictedTab";
@@ skipping 304 matching lines @@
 }
 
 void TabUsageRecorder::ClearDeletedTabs() {
   base::WeakNSObject<Tab> empty_tab(nil);
   auto tab_item = evicted_tabs_.find(empty_tab);
   while (tab_item != evicted_tabs_.end()) {
     evicted_tabs_.erase(empty_tab);
     tab_item = evicted_tabs_.find(empty_tab);
   }
 }