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

third_party/WebKit/Source/web/WebViewFrameWidget.cpp

 // 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.
 
 #include "web/WebViewFrameWidget.h"
 
 #include "web/WebInputMethodControllerImpl.h"
 #include "web/WebLocalFrameImpl.h"
 #include "web/WebViewImpl.h"
 
 namespace blink {
 
-WebViewFrameWidget::WebViewFrameWidget(WebWidgetClient* client,
+WebViewFrameWidget::WebViewFrameWidget(WebWidgetClient& client,
                                        WebViewImpl& webView,
                                        WebLocalFrameImpl& mainFrame)
-    : m_client(client), m_webView(&webView), m_mainFrame(&mainFrame) {
+    : m_client(&client), m_webView(&webView), m_mainFrame(&mainFrame) {
   m_mainFrame->setFrameWidget(this);
   m_webView->setCompositorVisibility(true);
 }
 
 WebViewFrameWidget::~WebViewFrameWidget() {}
 
 void WebViewFrameWidget::close() {
   // Note: it's important to use the captured main frame pointer here. During
   // a frame swap, the swapped frame is detached *after* the frame tree is
   // updated. If the main frame is being swapped, then
@@ skipping 196 matching lines @@
 
 CompositorAnimationHost* WebViewFrameWidget::animationHost() const {
   return m_webView->animationHost();
 }
 
 HitTestResult WebViewFrameWidget::coreHitTestResultAt(const WebPoint& point) {
   return m_webView->coreHitTestResultAt(point);
 }
 
 }  // namespace blink