MacViews: Implement Window-modal dialogs using sheets

ui/views/cocoa/views_nswindow_delegate.mm

Index: ui/views/cocoa/views_nswindow_delegate.mm
diff --git a/ui/views/cocoa/views_nswindow_delegate.mm b/ui/views/cocoa/views_nswindow_delegate.mm
index 38a56c1a17bbe06df96b2812bc90982a9458a2b8..6a344a38fa330e02742142f85982dc5cdfd6ee76 100644
--- a/ui/views/cocoa/views_nswindow_delegate.mm
+++ b/ui/views/cocoa/views_nswindow_delegate.mm
@@ -47,6 +47,13 @@
   parent_->OnVisibilityChangedTo(true);
 }
 
+- (void)sheetDidEnd:(NSWindow*)sheet
+         returnCode:(NSInteger)returnCode
+        contextInfo:(void*)contextInfo {
+  [sheet orderOut:nil];
+  parent_->OnWindowWillClose();
+}
+
 // NSWindowDelegate implementation.
 
 - (void)windowDidFailToEnterFullScreen:(NSWindow*)window {