This is particularly useful for knowing when it's safe to query for style and
layout information for a window without causing a synchronous style or layout
flush.
Note that promiseDocumentFlushed was chosen over promiseDidRefresh or promiseRefreshed
to avoid potential confusion with the actual network-level refresh of browsers or
documents.
MozReview-Commit-ID: Am3G9yvSgdN
--HG--
extra : rebase_source : 20bdd2d6f624767d919d95a6601fc1c890aadf10
a7df06446a