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According to https://drafts.csswg.org/css-syntax/#serializing-anb, if a
is 1 or -1 the number should be omitted. We did not follow this, causing
us to fail parts of external/wpt/css/cssom/selectorSerialize.html.
Change our behavior to match the spec and the test.

Bug: 818475
Change-Id: Ie0f44e2a3fb51069aad492096138020b67359982
Reviewed-on: https://chromium-review.googlesource.com/948066Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540827}