MacViews: Support -[NSWindow close] on sheets.

ui/views/widget/native_widget_mac.mm

 // Copyright 2014 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.
 
 #include "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include <utility>
 
@@ skipping 364 matching lines @@
   // does not do this. So post a regular task.
   base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindBlock(^{
     [window close];
   }));
 }
 
 void NativeWidgetMac::CloseNow() {
   if (!bridge_)
     return;
 
-  // Cocoa ignores -close calls on open sheets, so they should be closed
-  // asynchronously, using Widget::Close().
-  DCHECK(!IsWindowModalSheet());
-
   // NSWindows must be retained until -[NSWindow close] returns.
   base::scoped_nsobject<NSWindow> window(GetNativeWindow(),
                                          base::scoped_policy::RETAIN);
 
   // If there's a bridge at this point, it means there must be a window as well.
   DCHECK(window);
   [window close];
   // Note: |this| is deleted here when ownership_ == NATIVE_WIDGET_OWNS_WIDGET.
 }
 
@@ skipping 379 matching lines @@
   [[ViewsNSWindowCloseAnimator alloc] initWithWindow:window];
 }
 
 - (void)animationDidEnd:(NSAnimation*)animation {
   [window_ close];
   [animation_ setDelegate:nil];
   [self release];
 }
 
 @end