Bug 1622520 - only run Renderbuffer cleanup code when erased. r=jrmuizel
Bug 1622520 - only run Renderbuffer cleanup code when erased. r=jrmuizel

Differential Revision: https://phabricator.services.mozilla.com/D66868

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/6b4b41968cc165237c76b231d3ce5fcd46dc59ff

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