base: Fix/add header #ifndef guards.

base/test/null_task_runner.h

 // Copyright (c) 2012 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 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"
 
 namespace base {
 
 // Helper class for tests that need to provide an implementation of a
 // *TaskRunner class but don't actually care about tasks being run.
 
 class NullTaskRunner : public base::SingleThreadTaskRunner {
  public:
   NullTaskRunner();
 
   bool PostDelayedTask(const tracked_objects::Location& from_here,
                        const base::Closure& task,
                        base::TimeDelta delay) override;
   bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
                                   const base::Closure& task,
                                   base::TimeDelta delay) override;
   // Always returns true to avoid triggering DCHECKs.
   bool RunsTasksOnCurrentThread() const override;
 
  protected:
   ~NullTaskRunner() override;
 
   DISALLOW_COPY_AND_ASSIGN(NullTaskRunner);
 };
 
 }  // namespace base
+
+#endif  // BASE_TEST_NULL_TASK_RUNNER_H_