[DL]: Make getComputedStyle on locked subtree force update if needed

Fixes bug where we might not update the style values when only nodes
in locked subtrees are style-dirty (thus not marking the document
as needing update, which was needed to force the style update on
getComputedStyle calls) and adds test.

Bug: 882663, 963062
Change-Id: I1e04374430fec7fb78d5785fa1f19eced9d4afa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572984
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659807}
6 files changed