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In the presence of ::-webkit-scrollbar rules, author styling is applied
to scrollbars, but only if they actually contain declarations. Before
introducing lazy parsing of declaration blocks, we would not consider
empty style rules for matching. With lazy parsing, we do not necessarily
know if the rule is empty before we perform the actual parsing. That
caused a discrepancy between sheets from <style> elements and external
stylesheets for empty scrollbar rules because <style> element sheets do
not do lazy parsing of declaration blocks.

To make this consistent, check if a matched style rule is empty before
we mark the ComputedStyle as having pseudo element style.

Bug: 897100
Change-Id: I8b808271ca322f42b5ae5e5627a5d17a81658885
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874467Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708576}