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

ios/web/public/test/crw_mock_web_state_delegate.mm

Index: ios/web/public/test/crw_mock_web_state_delegate.mm
diff --git a/ios/web/public/test/crw_mock_web_state_delegate.mm b/ios/web/public/test/crw_mock_web_state_delegate.mm
index fda453cec2b682b52c82cb78a512e57fc078041a..e19725d053b167b2df24d8dd267468f35fd72e34 100644
--- a/ios/web/public/test/crw_mock_web_state_delegate.mm
+++ b/ios/web/public/test/crw_mock_web_state_delegate.mm
@@ -7,6 +7,10 @@
 #import "ios/web/public/web_state/context_menu_params.h"
 #import "ios/web/public/web_state/web_state.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation CRWMockWebStateDelegate {
   // Backs up the property with the same name.
   std::unique_ptr<web::WebState::OpenURLParams> _openURLParams;