[ObjC ARC] Converts ios/chrome/app/startup:startup to ARC.

ios/chrome/app/startup/register_experimental_settings.mm

Index: ios/chrome/app/startup/register_experimental_settings.mm
diff --git a/ios/chrome/app/startup/register_experimental_settings.mm b/ios/chrome/app/startup/register_experimental_settings.mm
index be142219f591db1cb6dcf14c19df7231424de373..5b0876c8627c694f8dae2711da9f9602449334d8 100644
--- a/ios/chrome/app/startup/register_experimental_settings.mm
+++ b/ios/chrome/app/startup/register_experimental_settings.mm
@@ -6,9 +6,12 @@
 
 #include "base/logging.h"
 #include "base/mac/bundle_locations.h"
-#import "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // Key in the UserDefaults for the Experimental Keys.
 NSString* kExperimentalKeysKey = @"ExperimentalKeys";
@@ -54,8 +57,7 @@ BOOL IsDefaultSettingValueValid(id value) {
   NSArray* settingsContent =
       [[NSFileManager defaultManager] contentsOfDirectoryAtPath:settingsFilepath
                                                           error:NULL];
-  base::scoped_nsobject<NSMutableArray> currentExpKeys(
-      [[NSMutableArray alloc] init]);
+  NSMutableArray* currentExpKeys = [[NSMutableArray alloc] init];
 
   for (NSString* filename in settingsContent) {
     // Only plist files are preferences definition.