Intent to ship: Frames timing functions

[Boris Chiou]

As of Firefox 55, I intend to land the patches of Frames timing functions
in Bug 1248340 <https://bugzilla.mozilla.org/show_bug.cgi?id=1248340>.
Currently, we decide not to add a pref for that, so we will ship it
directly. In this bug, we also contribute the tests into web-platform-test*,
*web-platform-tests/css-timing-1/*. *Other UAs haven't ship this
*.*
This feature was previously discussed in this "intent to implement" thread:
https://groups.google.com/forum/#!topic/mozilla.dev.platform/w-3HBnN3sc4

*Bug to turn on by default*:
https://bugzilla.mozilla.org/show_bug.cgi?id=1248340

*Link to standard*:
https://drafts.csswg.org/css-timing/#frames-timing-functions

--
Regards,
Boris