[ObjC ARC] Converts ios/net to ARC.

ios/net/cookies/cookie_store_ios_client.mm

Index: ios/net/cookies/cookie_store_ios_client.mm
diff --git a/ios/net/cookies/cookie_store_ios_client.mm b/ios/net/cookies/cookie_store_ios_client.mm
index c8cbd14c93ac3a83d364ee2c0fc4938c10019f30..d1b6e4c548987eb50347748e9718ebd1de3e8025 100644
--- a/ios/net/cookies/cookie_store_ios_client.mm
+++ b/ios/net/cookies/cookie_store_ios_client.mm
@@ -4,6 +4,10 @@
 
 #include "ios/net/cookies/cookie_store_ios_client.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // The CookieStoreIOSClient.
 net::CookieStoreIOSClient* g_client;