base: Fix/add header #ifndef guards.

base/test/null_task_runner.h

Index: base/test/null_task_runner.h
diff --git a/base/test/null_task_runner.h b/base/test/null_task_runner.h
index a6471f59cf77e94c22fe0a3784c8e8e0b6ea5494..0f447513c58d569628fe69586e739acae9db1154 100644
--- a/base/test/null_task_runner.h
+++ b/base/test/null_task_runner.h
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#ifndef BASE_TEST_NULL_TASK_RUNNER_H_
+#define BASE_TEST_NULL_TASK_RUNNER_H_
+
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/single_thread_task_runner.h"
@@ -31,3 +34,5 @@ class NullTaskRunner : public base::SingleThreadTaskRunner {
 };
 
 }  // namespace base
+
+#endif  // BASE_TEST_NULL_TASK_RUNNER_H_