…y ranges. r=kvark

The WR code that computed the sticky_offset didn't properly combine the offsets
from the top- and bottom- sticky calculations if an item had both. This patch
fixes the calculation, which makes the remaining test failure (in the
configuration without any dynamic toolbar) pass.

Depends on D70679

Differential Revision: https://phabricator.services.mozilla.com/D70680

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/2c065921de099b97532775e1aca5973edd5512b2