Bug 1218456 - Document::GetContainer shouldn't be virtual. r=smaug

It's never overridden. Also chances are we should remove it and just use
GetDocShell().

Differential Revision: https://phabricator.services.mozilla.com/D37405

--HG--
extra : moz-landing-system : lando

dom/base/Document.h

@@ -2339,7 +2339,7 @@ class Document : public nsINode,
   /**
    * Get the container (docshell) for this document.
    */
-  virtual nsISupports* GetContainer() const;
+  nsISupports* GetContainer() const;
 
   /**
    * Get the container's load context for this document.