Do proper snapshotting in TextOverflowPostLayoutSnapshot

We never used the snapshotted value in TextOverflowPostLayoutSnapshot
during layout, which meant we could be out of sync for the stored value
and the previous used value for forced layouts. That again lead to
invalidation issues.

Instead use the snapshotted value during layout. Also added some
documentation to the class.

Bug: 495842549
Change-Id: I6589385f01294811e484a53d5771254f48c9f46a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7698437
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1605344}