[ObjC ARC] Converts ios/web:ios_web_web_state_unittests to ARC.

ios/web/web_state/context_menu_params_utils_unittest.mm

Index: ios/web/web_state/context_menu_params_utils_unittest.mm
diff --git a/ios/web/web_state/context_menu_params_utils_unittest.mm b/ios/web/web_state/context_menu_params_utils_unittest.mm
index d43a4b352adef52d9cd99365010b48262b6c9b3e..838f468ae07df797e435054419ee0304c8bce533 100644
--- a/ios/web/web_state/context_menu_params_utils_unittest.mm
+++ b/ios/web/web_state/context_menu_params_utils_unittest.mm
@@ -13,6 +13,10 @@
 #import "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 {
 // Text values for the tapped element triggering the context menu.
 const char* kLinkUrl = "http://link.url/";