[CSS Grid Layout] Upgrade justify-content parsing to CSS3 Box Alignment spec.

Upgrade the justify-content property to the last CSS3 Box Alignment
specification. It defines a different enumeration for Positional
and Distribution alignment, which requires changes in the FexibleBox
impementation.

BUG=249451

Review URL: https://codereview.chromium.org/636993002

git-svn-id: svn://svn.chromium.org/blink/trunk@184660 bbb929c8-8fbe-4397-9dbb-9b2b20218538