Broaden justify-self's parsing name

This is in preparation of migrating align-self, align-items
and justify-items to the CSS 3 Alignment syntax.

The current naming was too tied to justify-self and needs to
be broadened. This will reduce the follow-up implementations'
patches.

BUG=249451

Review URL: https://codereview.chromium.org/131773002

git-svn-id: svn://svn.chromium.org/blink/trunk@164817 bbb929c8-8fbe-4397-9dbb-9b2b20218538