Use BisonCSSParser::parseColor to parses brand-color as we want to support all CSS color value.

Also, add test case to verify first brand-color meta tag takes effect.
and the brand-color can be changed by script.

BUG=383941

Review URL: https://codereview.chromium.org/341273008

git-svn-id: svn://svn.chromium.org/blink/trunk@176688 bbb929c8-8fbe-4397-9dbb-9b2b20218538