Returns the stack depth at which this monitor was
acquired by the owning thread. Returns -1 if the
implementation cannot determine the stack depth
(e.g., for monitors acquired by JNI MonitorEnter).
Returns:
the stack depth at which this monitor was
acquired by the owning thread.
Throws:
InvalidStackFrameException - if the associated stack
frame has become invalid. Once the frame's thread is resumed,
the stack frame is no longer valid.