[MacViews] Position modal sheets using ModalDialogHost.

ui/views/widget/native_widget_mac.h

Index: ui/views/widget/native_widget_mac.h
diff --git a/ui/views/widget/native_widget_mac.h b/ui/views/widget/native_widget_mac.h
index b050a4be2a0b54dc41765ae5cc43ef6f15a50e71..00af8020ada75f781183c728c1fc70be0c9bfdf5 100644
--- a/ui/views/widget/native_widget_mac.h
+++ b/ui/views/widget/native_widget_mac.h
@@ -36,6 +36,10 @@ class VIEWS_EXPORT NativeWidgetMac : public internal::NativeWidgetPrivate {
   // override this method for an early hook into the native window teardown.
   virtual void OnWindowWillClose();
 
+  // Returns the vertical position that sheets should be anchored, in pixels
+  // from the bottom of the window.
+  virtual int SheetPositionY();
+
   // internal::NativeWidgetPrivate:
   void InitNativeWidget(const Widget::InitParams& params) override;
   NonClientFrameView* CreateNonClientFrameView() override;