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

ios/chrome/browser/ui/UIView+SizeClassSupport.mm

 // Copyright 2015 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/UIView+SizeClassSupport.h"
 
 #import "base/logging.h"
 #import "base/ios/ios_util.h"
 #import "ios/chrome/browser/ui/ui_util.h"
 
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
 namespace {
 
 // Returns the SizeClassIdiom corresponding with |size_class|.
 SizeClassIdiom GetSizeClassIdiom(UIUserInterfaceSizeClass size_class) {
   switch (size_class) {
     case UIUserInterfaceSizeClassCompact:
       return COMPACT;
     case UIUserInterfaceSizeClassRegular:
       return REGULAR;
     case UIUserInterfaceSizeClassUnspecified:
@@ skipping 20 matching lines @@
 
 - (SizeClassIdiom)cr_heightSizeClass {
   UIWindow* keyWindow = [UIApplication sharedApplication].keyWindow;
   UIUserInterfaceSizeClass sizeClass = self.traitCollection.verticalSizeClass;
   if (!IsSizeClassSpecified(sizeClass))
     sizeClass = keyWindow.traitCollection.verticalSizeClass;
   return GetSizeClassIdiom(sizeClass);
 }
 
 @end