[MacViews] Position modal sheets using ModalDialogHost.

chrome/browser/ui/views/apps/app_window_native_widget_mac.h

 // Copyright 2015 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.
 
 #ifndef CHROME_BROWSER_UI_VIEWS_APPS_APP_WINDOW_NATIVE_WIDGET_MAC_H
 #define CHROME_BROWSER_UI_VIEWS_APPS_APP_WINDOW_NATIVE_WIDGET_MAC_H
 
 #include "ui/views/widget/native_widget_mac.h"
 
 namespace extensions {
 class NativeAppWindow;
 }
 
 // This implements features specific to app windows, e.g. frameless windows that
 // behave like normal windows.
 class AppWindowNativeWidgetMac : public views::NativeWidgetMac {
  public:
   AppWindowNativeWidgetMac(views::Widget* widget,
                            extensions::NativeAppWindow* native_app_window);
   ~AppWindowNativeWidgetMac() override;
 
+  // NativeWidgetMac:
+  int SheetPositionY() override;
+
  protected:
   // NativeWidgetMac:
   gfx::NativeWindow CreateNSWindow(
       const views::Widget::InitParams& params) override;
 
  private:
   // Weak. Owned by extensions::AppWindow (which manages our Widget via its
   // WebContents).
   extensions::NativeAppWindow* native_app_window_;
 
   DISALLOW_COPY_AND_ASSIGN(AppWindowNativeWidgetMac);
 };
 
 #endif  // CHROME_BROWSER_UI_VIEWS_APPS_APP_WINDOW_NATIVE_WIDGET_MAC_H