[UI] Make auto-resizable constrained web dialog work even when initiated

components/constrained_window/constrained_window_views.h

Index: components/constrained_window/constrained_window_views.h
diff --git a/components/constrained_window/constrained_window_views.h b/components/constrained_window/constrained_window_views.h
index d939f2d6e561a632a656583990e7859e917e5c0e..9a8b3e95f5f3f02c14387e605dff21c40a097ce4 100644
--- a/components/constrained_window/constrained_window_views.h
+++ b/components/constrained_window/constrained_window_views.h
@@ -42,6 +42,10 @@ void UpdateWidgetModalDialogPosition(
     views::Widget* widget,
     web_modal::ModalDialogHost* dialog_host);
 
+// Returns the top level WebContents of |initiator_web_contents|.
+content::WebContents* GetTopLevelWebContents(
+    content::WebContents* initiator_web_contents);
+
 // Shows the dialog with a new SingleWebContentsDialogManager. The dialog will
 // notify via WillClose() when it is being destroyed.
 void ShowModalDialog(gfx::NativeWindow dialog,