This commit adds two new telemetry probes to collect:
1. The amount of scroll anchoring adjustments applied
2. The total absolute length in CSS pixels of scroll anchoring adjustments
Both of these metrics are collected on a per top-level-document basis, and
reported with other use-counters.
Differential Revision: https://phabricator.services.mozilla.com/D16271
--HG--
extra : source : 43081bb8f4eac598a3103753965d5b60dfd1c3fa
extra : amend_source : 3d53dc56616f9e48799b7d8b8c11e96007d3c6eb
extra : intermediate-source : 6da6a7ff2d794c25b61858d16fe97e5f309e0f1f
c71371cf2a