[css-text-decor] Serialize text-decoration-line in canonical order

text-decoration-line's keywords (none, underline, overline,
line-through, blink) now serialize in canonical order:
https://drafts.csswg.org/css-text-decor-3/#text-decoration-line-property

Passing most of the tests in text-decoration-line-valid.html
except the ones related to unsupported keywords.
Updated test results for external/wpt/editing/run/underline.html.

BUG=952860
TEST=external/wpt/css/css-text-decor/parsing/text-decoration-line-valid.html

Change-Id: I0bf5a764858cbb1ea7722616a44b3b891a6c919f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1567768
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#651195}
7 files changed