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

third_party/WebKit/public/web/WebWidgetClient.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 40 matching lines @@
 class WebNode;
 class WebString;
 class WebWidget;
 struct WebCursorInfo;
 struct WebFloatPoint;
 struct WebFloatRect;
 struct WebFloatSize;
 
 class WebWidgetClient {
  public:
+  virtual ~WebWidgetClient() {}
+
   // Called when a region of the WebWidget needs to be re-painted.
   virtual void didInvalidateRect(const WebRect&) {}
 
   // Attempt to initialize compositing view for this widget. If successful,
   // returns a valid WebLayerTreeView which is owned by the
   // WebWidgetClient.
   virtual WebLayerTreeView* initializeLayerTreeView() { return nullptr; }
 
   // FIXME: Remove all overrides of this.
   virtual bool allowsBrokenNullLayerTreeView() const { return false; }
@@ skipping 101 matching lines @@
   // is eanbled.  TODO(oshima): Update the comment when the
   // migration is completed.
   virtual void convertWindowToViewport(WebFloatRect* rect) {}
 
   // Called when a drag-and-drop operation should begin.
   virtual void startDragging(WebReferrerPolicy,
                              const WebDragData&,
                              WebDragOperationsMask,
                              const WebImage& dragImage,
                              const WebPoint& dragImageOffset) {}
-
- protected:
-  ~WebWidgetClient() {}
 };
 
 }  // namespace blink
 
 #endif