Support unset in CSSParserFastPaths

Support unset as a CSS wide keyword [1] and add some unit tests for it.

[1] https://www.w3.org/TR/css3-values/#common-keywords

Bug: 660273
Change-Id: Ic739055b83988017e11ff3968c1a0aad48073613
Reviewed-on: https://chromium-review.googlesource.com/663820
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Reviewed-by: meade_UTC10 <meade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501642}
2 files changed