We need a way to switch it on and off to compare the performance and power usage of various test cases.
The new pref is "webrender.enable-multithreading" and does not require a restart.

Differential Revision: https://phabricator.services.mozilla.com/D61589

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/a9dde10ff4b84810ca804e74caf30e221345d9ed