[ObjC ARC] Converts ios/chrome/app/application_delegate to ARC.

ios/chrome/app/application_delegate/memory_warning_helper.mm

Index: ios/chrome/app/application_delegate/memory_warning_helper.mm
diff --git a/ios/chrome/app/application_delegate/memory_warning_helper.mm b/ios/chrome/app/application_delegate/memory_warning_helper.mm
index fa92f751969b97eef761a29331b8a2e080afe2a5..d8eeb93490bbe3d18a7db898a36dce7402099ff5 100644
--- a/ios/chrome/app/application_delegate/memory_warning_helper.mm
+++ b/ios/chrome/app/application_delegate/memory_warning_helper.mm
@@ -9,6 +9,10 @@
 #include "ios/chrome/browser/crash_report/breakpad_helper.h"
 #import "ios/chrome/browser/metrics/previous_session_info.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // The number of seconds to wait after a memory warning to clear the flag used
 // to detect Out Of Memory crashes.