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

ios/chrome/browser/ios_chrome_io_thread_unittest.mm

Index: ios/chrome/browser/ios_chrome_io_thread_unittest.mm
diff --git a/ios/chrome/browser/ios_chrome_io_thread_unittest.mm b/ios/chrome/browser/ios_chrome_io_thread_unittest.mm
index 692532a6304496c8887d65401cefd612a09ccdc0..379c30cf186405bd1ec8d3143ec89d4ded81826a 100644
--- a/ios/chrome/browser/ios_chrome_io_thread_unittest.mm
+++ b/ios/chrome/browser/ios_chrome_io_thread_unittest.mm
@@ -21,6 +21,10 @@
 #include "testing/gtest_mac.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // A delegate interface for users of URLFetcher.