Make the WebWidgetClient parameter a ref so it's not legal to pass a nullptr.

third_party/WebKit/public/web/WebViewClient.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 263 matching lines @@
   WebLayerTreeView* initializeLayerTreeView() override { return nullptr; }
   WebScreenInfo screenInfo() override { return WebScreenInfo(); }
   void setTouchAction(WebTouchAction touchAction) override {}
   void showUnhandledTapUIIfNeeded(const WebPoint& tappedPosition,
                                   const WebNode& tappedNode,
                                   bool pageChanged) override {}
   void show(WebNavigationPolicy) override {}
   virtual WebWidgetClient* widgetClient() { return this; }
 
  protected:
-  ~WebViewClient() {}
+  ~WebViewClient() override {}
 };
 
 }  // namespace blink
 
 #endif