Remove DocumentStyleSheetCollection and ShadowTreeStyleSheetCollection.

The differences between these two are minimal, which we can easily
deal with using a couple of ifs.

Note that this also makes the shadow tree case include a test for
skipping still-loading stylesheets that appear to be missing by
mistake (although one would believe it is an edge case).

Change-Id: I51d6f51d0f9a2196e074af39400dcbb4942f1920
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6832511
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1499605}