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

ios/chrome/browser/ui/tab_switcher/tab_model_snapshot.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.
 
 #import "ios/chrome/browser/ui/tab_switcher/tab_model_snapshot.h"
 
 #include "base/strings/sys_string_conversions.h"
 #import "ios/chrome/browser/tabs/tab.h"
+#import "ios/chrome/browser/tabs/tab_model.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
 
 TabModelSnapshot::TabModelSnapshot(TabModel* tabModel) {
   for (Tab* tab in tabModel) {
     _hashes.push_back(hashOfTheVisiblePropertiesOfATab(tab));
     _tabs.push_back(base::WeakNSObject<Tab>(tab));
   }
 }
 
 TabModelSnapshot::~TabModelSnapshot() {}
 
 std::vector<size_t> const& TabModelSnapshot::hashes() const {
   DCHECK_EQ(_tabs.size(), _hashes.size());
   return _hashes;
 }
 
 std::vector<base::WeakNSObject<Tab>> const& TabModelSnapshot::tabs() const {
   DCHECK_EQ(_tabs.size(), _hashes.size());
   return _tabs;
 }
 
 size_t TabModelSnapshot::hashOfTheVisiblePropertiesOfATab(Tab* tab) {
   DCHECK(tab);
   std::stringstream ss;
   // lastVisitedTimestamp is used as an approximation for whether the tab's
   // snapshot changed.
-  ss << tab.tabId << std::endl
+  ss << base::SysNSStringToUTF8(tab.tabId) << std::endl
      << base::SysNSStringToUTF8(tab.urlDisplayString) << std::endl
      << std::hexfloat << tab.lastVisitedTimestamp << std::endl;
   return std::hash<std::string>()(ss.str());
 }