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

content/shell/test_runner/web_widget_test_client.h

 // Copyright 2016 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 CONTENT_SHELL_TEST_RUNNER_WEB_WIDGET_TEST_CLIENT_H_
 #define CONTENT_SHELL_TEST_RUNNER_WEB_WIDGET_TEST_CLIENT_H_
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "third_party/WebKit/public/web/WebWidgetClient.h"
 
 namespace test_runner {
 
 class TestRunner;
 class TestRunnerForSpecificView;
 class WebTestDelegate;
 class WebWidgetTestProxyBase;
 
 // WebWidgetTestClient implements WebWidgetClient interface, providing behavior
 // expected by tests.  WebWidgetTestClient ends up used by WebViewTestProxy
 // which coordinates forwarding WebWidgetClient calls either to
 // WebWidgetTestClient or to the product code (i.e. currently to
 // RenderViewImpl).
 class WebWidgetTestClient : public blink::WebWidgetClient {
  public:
   // Caller has to ensure that all arguments (i.e. |test_runner| and |delegate|)
   // live longer than |this|.
   WebWidgetTestClient(WebWidgetTestProxyBase* web_widget_test_proxy_base);
 
-  virtual ~WebWidgetTestClient();
+  ~WebWidgetTestClient() override;
 
   // WebWidgetClient overrides needed by WebWidgetTestProxy.
   blink::WebScreenInfo screenInfo() override;
   void scheduleAnimation() override;
   bool requestPointerLock() override;
   void requestPointerUnlock() override;
   bool isPointerLocked() override;
   void setToolTipText(const blink::WebString& text,
                       blink::WebTextDirection direction) override;
   void startDragging(blink::WebReferrerPolicy policy,
@@ skipping 15 matching lines @@
   bool animation_scheduled_;
 
   base::WeakPtrFactory<WebWidgetTestClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WebWidgetTestClient);
 };
 
 }  // namespace test_runner
 
 #endif  // CONTENT_SHELL_TEST_RUNNER_WEB_WIDGET_TEST_CLIENT_H_