ppb_message_loop: rename _nested to _int
ppb_message_loop: add _get_depth()
ppb_message_loop: honor depth in _post_quit_depth()

post_quit() intended for some outerloop could make newly created loop
to quit early. Taking desired loop depth into account will address the
issue.
use ppb_message_loop_post_quit_depth() to avoid layering errors
np_functions: allocate message loop for browser thread only once
rename files, remove _dev, _private, _trusted suffixes