MacViews: Support -[NSWindow close] on sheets.

ui/views/widget/native_widget_mac.mm

Index: ui/views/widget/native_widget_mac.mm
diff --git a/ui/views/widget/native_widget_mac.mm b/ui/views/widget/native_widget_mac.mm
index 1c6b46af73aa14cc9f57f8c992814b6d89d6e700..7da431fc0337d2b79b46c88c11906434db84fcd6 100644
--- a/ui/views/widget/native_widget_mac.mm
+++ b/ui/views/widget/native_widget_mac.mm
@@ -382,10 +382,6 @@ void NativeWidgetMac::CloseNow() {
   if (!bridge_)
     return;
 
-  // Cocoa ignores -close calls on open sheets, so they should be closed

[tapted, 2017/01/13 19:36:53]

I think when I wrote this things were structured a bit different and the problem
was that -close wasn't triggering sheetDidEnd: . Hopefully I've catered for all
the dragons now.. (but this should only come up in tests and during process
shutdown anyway, since having a sheet will disable the close button for users).

-  // asynchronously, using Widget::Close().
-  DCHECK(!IsWindowModalSheet());
-
   // NSWindows must be retained until -[NSWindow close] returns.
   base::scoped_nsobject<NSWindow> window(GetNativeWindow(),
                                          base::scoped_policy::RETAIN);