Allow a Views client to provide a default frameview for window widgets.

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
===================================================================
--- ui/views/widget/widget.cc	(revision 117091)
+++ ui/views/widget/widget.cc	(working copy)
@@ -750,6 +750,10 @@
   NonClientFrameView* frame_view = widget_delegate_->CreateNonClientFrameView();
   if (!frame_view)
     frame_view = native_widget_->CreateNonClientFrameView();
+  if (!frame_view && ViewsDelegate::views_delegate) {
+    frame_view =
+        ViewsDelegate::views_delegate->CreateDefaultNonClientFrameView(this);
+  }
   return frame_view ? frame_view : new CustomFrameView(this);
 }