It used to be declared as ulint, which is 32/64 bit integer depending on OS bitness.
However this declaration lead to problems with os_compare_and_swap_thread_id 
implementation which operated on 32 bit integers (InterlockedCompareExchange)

Using 64 bit int   lead to  wrong result in os_compare_and_swap_thread_id 
and to crash/assertion in  rw_lock_set_writer_id_and_recursion_flag()

The fix is to use 4 byte integer to os_thread_id type definition.