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

ios/chrome/browser/ui/util/snapshot_util.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/util/snapshot_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace snapshot_util {
 
 UIView* GenerateSnapshot(UIView* view) {
   UIView* snapshot;
   if (view.window) {
     // |view| is in a view hierarchy.
     snapshot = [view snapshotViewAfterScreenUpdates:NO];
   } else {
     UIGraphicsBeginImageContextWithOptions(view.bounds.size, NO, 0);
     [view.layer renderInContext:UIGraphicsGetCurrentContext()];
     UIImage* screenshot = UIGraphicsGetImageFromCurrentImageContext();
     UIGraphicsEndImageContext();
-    snapshot = [[[UIView alloc] initWithFrame:CGRectZero] autorelease];
+    snapshot = [[UIView alloc] initWithFrame:CGRectZero];
     snapshot.layer.contents = static_cast<id>(screenshot.CGImage);
   }
   return snapshot;
 }
 
 }  // namespace snapshot_util