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

ios/chrome/browser/ui/main/main_view_controller.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/main/main_view_controller.h"
 
 #import "base/logging.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation MainViewController
 
 - (UIViewController*)activeViewController {
   return [self.childViewControllers firstObject];
 }
 
 - (void)setActiveViewController:(UIViewController*)activeViewController {
   DCHECK(activeViewController);
   if (self.activeViewController == activeViewController)
     return;
@@ skipping 48 matching lines @@
   return self.activeViewController;
 }
 
 - (BOOL)shouldAutorotate {
   return self.activeViewController
              ? [self.activeViewController shouldAutorotate]
              : [super shouldAutorotate];
 }
 
 @end