Bug 1554426 - Re-introduce Renderer::current_epoch. r=emilio
Bug 1554426 - Re-introduce Renderer::current_epoch. r=emilio

It was removed in 1441308 but is used by servo

Differential Revision: https://phabricator.services.mozilla.com/D32598

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/d3bb565468f694e71c793020b47f679f5a8b860c

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender