[ObjC ARC] Converts ios/chrome/browser/signin:signin to ARC.

ios/chrome/browser/signin/signin_util.mm

Index: ios/chrome/browser/signin/signin_util.mm
diff --git a/ios/chrome/browser/signin/signin_util.mm b/ios/chrome/browser/signin/signin_util.mm
index a8d66e7c404e021f809a97e7ada203c2ed170d2d..5babbb72a1e675fdf99933513f9609c1f64ce6e5 100644
--- a/ios/chrome/browser/signin/signin_util.mm
+++ b/ios/chrome/browser/signin/signin_util.mm
@@ -10,8 +10,12 @@
 #import "ios/public/provider/chrome/browser/signin/chrome_identity.h"
 #include "ios/public/provider/chrome/browser/signin/signin_error_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 NSArray* GetScopeArray(const std::set<std::string>& scopes) {
-  NSMutableArray* scopes_array = [[[NSMutableArray alloc] init] autorelease];
+  NSMutableArray* scopes_array = [[NSMutableArray alloc] init];
   for (const auto& scope : scopes) {
     [scopes_array addObject:base::SysUTF8ToNSString(scope)];
   }