Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/test/ios_chrome_unit_test_suite.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 IOS_CHROME_TEST_IOS_CHROME_UNIT_TEST_SUITE_H_
 #define IOS_CHROME_TEST_IOS_CHROME_UNIT_TEST_SUITE_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/single_thread_task_runner.h"
 #include "ios/web/public/test/web_test_suite.h"
 
 // Test suite for unit tests.
 class IOSChromeUnitTestSuite : public web::WebTestSuite {
  public:
   IOSChromeUnitTestSuite(int argc, char** argv);
   ~IOSChromeUnitTestSuite() override;
 
   // web::WebTestSuite overrides:
   void Initialize() override;
 
  private:
+  scoped_refptr<base::SingleThreadTaskRunner> action_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(IOSChromeUnitTestSuite);
 };
 
 #endif  // IOS_CHROME_TEST_IOS_CHROME_UNIT_TEST_SUITE_H_