[ObjC ARC] Converts ios/chrome/browser/ui/tab_switcher:tab_switcher to ARC.

ios/chrome/browser/ui/tab_switcher/tab_switcher_session_changes.mm

 // Copyright 2016 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/ui/tab_switcher/tab_switcher_session_changes.h"
 
 #import "ios/chrome/browser/ui/tab_switcher/tab_switcher_utils.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 TabSwitcherSessionChanges::TabSwitcherSessionChanges(
     std::vector<size_t> const& tabHashesInInitialState,
     std::vector<size_t> const& tabHashesInFinalState) {
   TabSwitcherMinimalReplacementOperations(tabHashesInInitialState,
                                           tabHashesInFinalState, &updates_,
                                           &deletions_, &insertions_);
 }
 
 TabSwitcherSessionChanges::~TabSwitcherSessionChanges() {}
 
 bool TabSwitcherSessionChanges::HasChanges() const {
   return updates_.size() || deletions_.size() || insertions_.size();
 }