ppb_instance_private_execute_script: run nested loop
ppb_instance: ensure _ptac called when loop is nested
ppb_message_loop: print errors
n2p_proxy: convert waiting barriers to nested loops
ppb_instance: acquire window_obj only once, on instance creation
ppb_url_util_dev_get_document_url: use nested loop for waiting
ppb_url_loader: use nested loops to wait for result
add browser thread support to ppb_message_loop
hide npn.pluginthreadasynccall behind ppb_core_call_on_browser_thread