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This patch implements proper parsing of syntax strings in the P&V API,
as well parsing of the actual values. In particular, we add support for
parsing of syntax strings like "<length> | <percentage>" and "<number>+".

- Whitespace is not mentioned in the spec but used in examples. This
patch allows whitespace in most places.
- The <ident> production is possibly not the best choice (see github
issue below), so until that is resolved we only allow name code points.
- <resolution> is not yet used in any CSS properties in Blink, so value
parsing support will be added separately.
- <transform-function> isn't factored to be trivially supported, so value
parsing support will also be added separately. There's also an
outstanding issue re. Typed OM integration which might result in changes
here.

https://drafts.css-houdini.org/css-properties-values-api/#supported-syntax-strings
https://github.com/w3c/css-houdini-drafts/issues/265

BUG=641877

Review-Url: https://codereview.chromium.org/2330893002
Cr-Commit-Position: refs/heads/master@{#419425}