This doesn't introduce any functional changes. However, it refactors
the way that stacking context coords are converted into reference
frame relative coordinates.
Having a single method to retrieve the current offset will make it
easier to take advantage of the newly added API that allows Gecko
to supply initial scroll offsets for scroll nodes. In turn, this
will allow WR to normalize the local coordinates of primitives, which
will allow future improvements and simplifications to the picture
caching implementation.
Differential Revision: https://phabricator.services.mozilla.com/D21090
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extra : moz-landing-system : lando