[ObjC ARC] Converts ios/net to ARC.

ios/net/cookies/cookie_creation_time_manager.mm

Index: ios/net/cookies/cookie_creation_time_manager.mm
diff --git a/ios/net/cookies/cookie_creation_time_manager.mm b/ios/net/cookies/cookie_creation_time_manager.mm
index 48a90eaf650714c9ff8f90cf4e7ab9d54bcfb3d9..fb06d261479c50981ab8eaa6d032789f9403317b 100644
--- a/ios/net/cookies/cookie_creation_time_manager.mm
+++ b/ios/net/cookies/cookie_creation_time_manager.mm
@@ -11,6 +11,10 @@
 #include "base/strings/sys_string_conversions.h"
 #include "base/time/time.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // Key holding the creation-time in NSHTTPCookie properties.
 // This key is undocumented, and its value has type NSNumber.
 NSString* const kHTTPCookieCreated = @"Created";