Bug 1716579 - Minimize memory usage of windows that haven't been rendered for a long time. r=gfx-reviewers,bradwerth
Bug 1716579 - Minimize memory usage of windows that haven't been rendered for a long time. r=gfx-reviewers,bradwerth

Differential Revision: https://phabricator.services.mozilla.com/D117918

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/fb2f9cfe820d43a07e11a2bcb46c626ea80c91e1

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