Fix thread hang

- Primary problem was ScopedThreadListLock was releasing heap lock in constructor instead of destructor
- Secondary problem was ScopedThreadListLock should not be used with Mutex::Wait
- Added Thread.getStackTrace case to ThreadStress that reproduces YouTube problem
- Added Mutex::GetDepth and related methods that were useful in diagnoising this issue

Change-Id: I1bdc7245e9b411378b98f4dcf498ad66eb96366d
diff --git a/src/thread_list.h b/src/thread_list.h
index 5da9877..eb08401 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -92,6 +92,7 @@
   ~ScopedThreadListLock();
 
  private:
+  bool heap_lock_held_;
   DISALLOW_COPY_AND_ASSIGN(ScopedThreadListLock);
 };