Linux sandbox: Provide AssertSingleThreaded() helper

sandbox/linux/services/thread_helpers.h

Index: sandbox/linux/services/thread_helpers.h
diff --git a/sandbox/linux/services/thread_helpers.h b/sandbox/linux/services/thread_helpers.h
index 377742a539acd4abb169ca400c0dcfabc3a74c37..88360afb567f108f4f7ebd7cfb1af3b78616a595 100644
--- a/sandbox/linux/services/thread_helpers.h
+++ b/sandbox/linux/services/thread_helpers.h
@@ -21,12 +21,22 @@ class SANDBOX_EXPORT ThreadHelpers {
   // crash if it cannot.
   static bool IsSingleThreaded(int proc_self_task);
 
+  // Crash if the current process is not single threaded. This will wait
+  // on /proc to be updated. In the case where this doesn't crash, this will
+  // return promptly. In the case where this does crash, this will first wait
+  // for a few ms in Debug mode, a few seconds in Release mode.
+  // If |proc_self_tasks| is -1, this method will open /proc/self/task/ and
+  // crash if it cannot.
+  static void AssertSingleThreaded(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,
+  static bool StopThreadAndWatchProcFS(int proc_self_task,
                                        base::Thread* thread);
 
+  static const char* GetAssertSingleThreadedErrorMessageForTests();
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadHelpers);
 };