Intent to unship: -moz-prefixed Page Visibility API.

[Thomas Wisniewski]

I intend to remove the prefixed version of the Page Visibility API,
which consists of the mozvisibilitychange event and mozHidden and
mozVisibilityState document properties.

The unprefixed versions of these features have been available since
Firefox 18, and a deprecation warning was issued on fxsitecompat.com
in October 2015. All other engines have supported the unprefixed
version for quite a while as well (IE11, Chrome 33, Safari 6.1).

Bug for removal: https://bugzilla.mozilla.org/show_bug.cgi?id=812701
Deprecation notice:
https://www.fxsitecompat.com/en-CA/docs/2015/prefixed-page-visibility-api-will-be-removed/
Link to standard: https://www.w3.org/TR/page-visibility/