…t r=gw,nical

Fixes an issue when backdrop-filter is used many time on a page where we would
spend a large amount of time reevaluating render tasks when assigning task depths.

Differential Revision: https://phabricator.services.mozilla.com/D41252

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/0c820a515e16953b3945ac670dac13099a568f6a