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

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

Index: ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.mm
diff --git a/ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.mm b/ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.mm
index 1ccdee36cf395fc73f40a94568838545d6b51859..debfedaa1a8cb62a627eec79031a3582dc9858d5 100644
--- a/ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.mm
+++ b/ios/chrome/browser/ui/tab_switcher/tab_switcher_utils_unittest.mm
@@ -6,6 +6,10 @@
 
 #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|