Remove OffsetForInFlowPosition() from PaintLayer::location_

The reason to omit OffsetForInFlowPosition is that it can
depend on sticky position constraints and scroll offset.
Depending on sticky position constraints is illogical, because
those constraints are updated during the CompositingInputsUpdater.
Depending on scroll is bad because it means we have to update
location_ every time scroll offset changes, for sticky elements.

OffsetForInFlowPosition can instead be computed dynamically in all
callsites of PaintLayer::Location(). The computational cost of doing
so should be cheap relative to the algorithms that call Location().

One side-effect is that this patch entails a partial revert of
https://chromium-review.googlesource.com/c/chromium/src/+/1686597.
This is because we need to cache OffsetForInFlowPosition for rel-pos
elements, in order to detect their movement and invalidate
compositing inputs if so (the location of a composited layer depends
on its PaintLayer's position). It should be possible to keep
track of whether OffsetForInFlowPosition changed during layout itself,
but that is left for a future patch.

And now UpdateLayerPositionsAfterOverflowScroll is not needed so that
method is removed.

Bug: 980767
Change-Id: I4ebc54846ddc041713d9571f17c4f6add97f4cba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700337
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677360}