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

third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp

 /*
  * Copyright (C) 2014 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 64 matching lines @@
 #include "wtf/AutoReset.h"
 #include "wtf/PtrUtil.h"
 #include <memory>
 
 namespace blink {
 
 // WebFrameWidget ------------------------------------------------------------
 
 WebFrameWidget* WebFrameWidget::create(WebWidgetClient* client,
                                        WebLocalFrame* localRoot) {
+  DCHECK(client) << "A valid WebWidgetClient must be supplied.";
   // Pass the WebFrameWidget's self-reference to the caller.
   return WebFrameWidgetImpl::create(client, localRoot);
 }
 
 WebFrameWidget* WebFrameWidget::create(WebWidgetClient* client,
                                        WebView* webView,
                                        WebLocalFrame* mainFrame) {
-  return new WebViewFrameWidget(client, toWebViewImpl(*webView),
+  DCHECK(client) << "A valid WebWidgetClient must be supplied.";
+  return new WebViewFrameWidget(*client, toWebViewImpl(*webView),
                                 toWebLocalFrameImpl(*mainFrame));
 }
 
 WebFrameWidgetImpl* WebFrameWidgetImpl::create(WebWidgetClient* client,
                                                WebLocalFrame* localRoot) {
+  DCHECK(client) << "A valid WebWidgetClient must be supplied.";
   // Pass the WebFrameWidgetImpl's self-reference to the caller.
   return new WebFrameWidgetImpl(
       client, localRoot);  // SelfKeepAlive is set in constructor.
 }
 
 WebFrameWidgetImpl::WebFrameWidgetImpl(WebWidgetClient* client,
                                        WebLocalFrame* localRoot)
     : m_client(client),
       m_localRoot(toWebLocalFrameImpl(localRoot)),
       m_mutator(nullptr),
@@ skipping 1030 matching lines @@
   return nullptr;
 }
 
 LocalFrame* WebFrameWidgetImpl::focusedLocalFrameAvailableForIme() const {
   if (!m_imeAcceptEvents)
     return nullptr;
   return focusedLocalFrameInWidget();
 }
 
 }  // namespace blink