Bug 1558271 - Update mozilla/browsertime snapshot to...

Bug 1558271 - Update mozilla/browsertime snapshot to b8c1becaee74970a6f6e4222a64d1e2e18f20cd6 r=nalexander

Changes include a fix for visual metrics calculations on desktop, and appending visual metrics information to gecko profiles.

Additionally, add the browsertime-results directory to gitignore.

Differential Revision: https://phabricator.services.mozilla.com/D34412

--HG--
extra : moz-landing-system : lando