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

content/shell/test_runner/web_view_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_VIEW_TEST_CLIENT_H_
 #define CONTENT_SHELL_TEST_RUNNER_WEB_VIEW_TEST_CLIENT_H_
 
 #include "base/macros.h"
 #include "third_party/WebKit/public/web/WebViewClient.h"
 
 namespace blink {
 class WebView;
 }  // namespace blink
 
 namespace test_runner {
 
 class TestRunner;
 class WebTestDelegate;
 class WebViewTestProxyBase;
 
 // WebViewTestClient implements WebViewClient interface, providing behavior
 // expected by tests.  WebViewTestClient ends up used by WebViewTestProxy
 // which coordinates forwarding WebViewClient calls either to
 // WebViewTestClient or to the product code (i.e. to RenderViewImpl).
 class WebViewTestClient : public blink::WebViewClient {
  public:
   // Caller has to ensure |web_view_test_proxy_base| lives longer than |this|.
   WebViewTestClient(WebViewTestProxyBase* web_view_test_proxy_base);
 
-  virtual ~WebViewTestClient();
+  ~WebViewTestClient() override;
 
   // WebViewClient overrides needed by WebViewTestProxy.
   void showValidationMessage(const blink::WebRect& anchor_in_root_view,
                              const blink::WebString& main_message,
                              blink::WebTextDirection main_message_hint,
                              const blink::WebString& sub_message,
                              blink::WebTextDirection sub_message_hint) override;
   void didChangeContents() override;
   blink::WebView* createView(blink::WebLocalFrame* creator,
                              const blink::WebURLRequest& request,
@@ skipping 13 matching lines @@
 
   // Borrowed pointer to WebViewTestProxyBase.
   WebViewTestProxyBase* web_view_test_proxy_base_;
 
   DISALLOW_COPY_AND_ASSIGN(WebViewTestClient);
 };
 
 }  // namespace test_runner
 
 #endif  // CONTENT_SHELL_TEST_RUNNER_WEB_VIEW_TEST_CLIENT_H_