Ensure that when themes in desktop chrome AURA on Windows use the opaque browser frame view.

ui/views/widget/desktop_aura/desktop_root_window_host_win.h

Index: ui/views/widget/desktop_aura/desktop_root_window_host_win.h
===================================================================
--- ui/views/widget/desktop_aura/desktop_root_window_host_win.h	(revision 181966)
+++ ui/views/widget/desktop_aura/desktop_root_window_host_win.h	(working copy)
@@ -200,6 +200,10 @@
   const Widget* GetWidget() const;
   HWND GetHWND() const;
 
+  const DesktopNativeWidgetAura* desktop_native_widget_aura() const {
+    return desktop_native_widget_aura_;
+  }
+
  private:
   // We are owned by the RootWindow, but we have to have a back pointer to it.
   aura::RootWindow* root_window_;