For color properties, we support 'currentcolor' as a CSSKeywordValue,
but any other value (including identifiers like 'red') are not supported
and are converted to base CSSStyleValues.

We add tests for most color related properties. We are failing the
text-emphasis-color one because we haven't unprefixed it yet.

We change the code to make this work.

Bug: 774887
Change-Id: Ide5463b9f67b63568ff32bd34e977faf16964bc1
Reviewed-on: https://chromium-review.googlesource.com/927943
Commit-Queue: Darren Shen <[email protected]>
Reviewed-by: nainar <[email protected]>
Cr-Commit-Position: refs/heads/master@{#538668}