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

third_party/WebKit/Source/web/WebViewFrameWidget.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be found
 // in the LICENSE file.
 
 #ifndef WebViewFrameWidget_h
 #define WebViewFrameWidget_h
 
 #include "platform/heap/Handle.h"
 #include "web/WebFrameWidgetBase.h"
 #include "web/WebInputMethodControllerImpl.h"
@@ skipping 18 matching lines @@
 // After the embedder starts using a WebFrameWidget for the main frame,
 // WebView will be updated to no longer inherit WebWidget. The eventual goal is
 // to unfork the widget code duplicated in WebFrameWidgetImpl and WebViewImpl
 // into one class.
 // A more detailed writeup of this transition can be read at
 // https://goo.gl/7yVrnb.
 class WebViewFrameWidget : public WebFrameWidgetBase {
   WTF_MAKE_NONCOPYABLE(WebViewFrameWidget);
 
  public:
-  explicit WebViewFrameWidget(WebWidgetClient*,
+  explicit WebViewFrameWidget(WebWidgetClient&,
                               WebViewImpl&,
                               WebLocalFrameImpl&);
   virtual ~WebViewFrameWidget();
 
   // WebFrameWidget overrides:
   void close() override;
   WebSize size() override;
   void resize(const WebSize&) override;
   void resizeVisualViewport(const WebSize&) override;
   void didEnterFullscreen() override;
@@ skipping 54 matching lines @@
 
  private:
   WebWidgetClient* m_client;
   RefPtr<WebViewImpl> m_webView;
   Persistent<WebLocalFrameImpl> m_mainFrame;
 };
 
 }  // namespace blink
 
 #endif  // WebViewFrameWidget_h