[CSS Grid Layout] Implement 'justify-self' parsing

This change adds the property from CSS 3 Box Alignment
and implements the parsing.

For now, the property is protected by the CSS Grid Layout
runtime flag as it is the only consumer but we may have
to introduce a new flag for the other layouts (block, flex,
...) as defined by the Alignment spec.

This change also fixes a bug uncovered in color parsing code,
where we would consider invalid colors as valid system colors.

BUG=249451

Review URL: https://codereview.chromium.org/17090005

git-svn-id: svn://svn.chromium.org/blink/trunk@164685 bbb929c8-8fbe-4397-9dbb-9b2b20218538