Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/test/ios_chrome_unit_test_suite.h

Index: ios/chrome/test/ios_chrome_unit_test_suite.h
diff --git a/ios/chrome/test/ios_chrome_unit_test_suite.h b/ios/chrome/test/ios_chrome_unit_test_suite.h
index 86e8314d9e193c56979d953c9b50c93d64e0d478..faf545dc66472a7011339e558bb9c83220177be5 100644
--- a/ios/chrome/test/ios_chrome_unit_test_suite.h
+++ b/ios/chrome/test/ios_chrome_unit_test_suite.h
@@ -7,6 +7,7 @@
 
 #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.
@@ -19,6 +20,8 @@ class IOSChromeUnitTestSuite : public web::WebTestSuite {
   void Initialize() override;
 
  private:
+  scoped_refptr<base::SingleThreadTaskRunner> action_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(IOSChromeUnitTestSuite);
 };