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

ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.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_utils.h"
 
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 class TabSwitcherUtilsTest : public PlatformTest {
  protected:
   // Checks that the computed updates/deletions/insertions to go from |initial|
   //  to |final| correctly match the expected updates/deletions/insertions.
   void TestLevenshtein(std::vector<size_t> const& initial,
                        std::vector<size_t> const& final,
                        std::vector<size_t> const& expectedSubstitutions,
                        std::vector<size_t> const& expectedDeletions,
                        std::vector<size_t> const& expectedInsertions) {
     std::vector<size_t> substitutions;
@@ skipping 98 matching lines @@
 TEST_F(TabSwitcherUtilsTest,
        TestLevenshteinPreferInsertionsAndDeletionsOverSubstitutions) {
   std::vector<size_t> initial = {0, 1};
   std::vector<size_t> final = {1, 2};
   std::vector<size_t> expectedSubstitutions = {};
   std::vector<size_t> expectedDeletions = {0};
   std::vector<size_t> expectedInsertions = {1};
   TestLevenshtein(initial, final, expectedSubstitutions, expectedDeletions,
                   expectedInsertions);
 }