ScrollAnchor: Only allow scroll anchor to be "just off screen"

During scroll anchoring, before layout, we determine an on-screen
anchor and then process the layout and ensure that one of the top
corners (left or right depending on writing mode) is positioned
in the same spot as before. However, this causes problems if the
size of the anchor also changes (with visibility) to be far off
screen. Instead, only allow the anchor to move "just off screen",
with one of its edges touching the viewport area.

This is only applied to content-visibility: auto elements that
have a pending layout.

The effect is similar: the object is moved off screen, but it prevents
unstable behavior with content-visibility: auto.

Note that the intent of scroll anchoring is to keep the visible
contents from shifting. However, in this case the contents are
shifting anyway (since the anchor is off screen), so manipulating
exactly how far off-screen should be ok. It is also limited to
only content-visibility auto elements.

R=chrishtr@chromium.org

Bug: 1138801
Change-Id: I1fbc5f8b634563c770db29bf30ef6d409d810614
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481348
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818681}