This removes the WebRender side of the previous slow frame indicator and replace it with a simple implementation that only looks at the CPU time on the render backend and renderer thread involved for building a frame.

A followup patch will add a separate indicator for when the displaylist/ipc/scene bits take too long.

Differential Revision: https://phabricator.services.mozilla.com/D69247

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/3ca010d7a52d68e7fbc2e91edaeb980cdd6e76fb