Bug 1484047 - Fix glitching of offset value. r=gw
Bug 1484047 - Fix glitching of offset value. r=gw

Based on the documentation of the `offset` field in a previous patch, it
doesn't make sense to be adjusting this field when getting a new display
list. The external_scroll_offset may change, but the user-visible offset
should be preserved.

This incorrect adjustment caused the `offset` field to hold an incorrect
value in the interval between applying the new display list and re-sampling
the async scroll delta from APZ. Although this would never appear to the
user in a composited frame, hit-tests during this interval could end up
returning the wrong result.

Differential Revision: https://phabricator.services.mozilla.com/D85333

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/6211bc3a4985732255d5ffc8b08e98bc12851e6a

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender