Linux sandbox: add basic Yama support

sandbox/linux/services/thread_helpers.h

 // Copyright 2014 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 SANDBOX_LINUX_SERVICES_THREAD_HELPERS_H_
 #define SANDBOX_LINUX_SERVICES_THREAD_HELPERS_H_
 
 #include "base/basictypes.h"
 
 namespace base { class Thread; }
 
 namespace sandbox {
 
 class ThreadHelpers {
  public:
   // Check whether the current process is single threaded. |proc_self_tasks|
-  // should be a file descriptor to /proc/self/task/ and remains owned by the
-  // caller.
+  // can be a file descriptor to /proc/self/task/ and remains owned by the
+  // caller or -1.
+  // If |proc_self_tasks| is -1, this method will open /proc/self/task/ and
+  // crash if it cannot.
   static bool IsSingleThreaded(int proc_self_task);
 
   // Stop |thread| and ensure that it does not have an entry in
   // /proc/self/task/ from the point of view of the current thread. This is
   // the way to stop threads before calling IsSingleThreaded().
   static bool StopThreadAndWatchProcFS(int proc_self_tasks,
                                        base::Thread* thread);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadHelpers);
 };
 
 }  // namespace content
 
 #endif  // SANDBOX_LINUX_SERVICES_THREAD_HELPERS_H_