[anchor] Make containing block logic clearer.

Previously we populated two variants of the ContainingBlockInfo,
one for abspos, and one for fixedpos.

Fixedpos only differed at the ICB as it used a slightly different size.

Instead of populating both these, make the fixedpos one std::optional
and only populate it when it differs.

There should be no behaviour change.

Change-Id: Ifd168f7dbf3c12964245871428a396c3bd6aed50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6897869
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508365}