Bug 1484197 [wpt PR 12547] - [css-properties-values-api] Support string values in...

Bug 1484197 [wpt PR 12547] - [css-properties-values-api] Support string values in StylePropertyMap.set., a=testonly

Automatic update from web-platform-tests[css-properties-values-api] Support string values in StylePropertyMap.set.

When producing a CSSStyleValue from a string for a registered custom
property, parse the value according to the syntax instead of always
creating a CSSUnparsedValue.

Note that <color> and <url> are still not supported, because
CSSUnsupportedProperty does not work properly for registered custom
properties (yet).

R=futhark@chromium.org

Bug: 641877
Change-Id: I806eb5b0c5112956e34457808791367423587c76
Reviewed-on: https://chromium-review.googlesource.com/1177755
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587537}

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wpt-commits: d9bcf59e3942bdbfd165e1010acc0ad1427491ba
wpt-pr: 12547