Properly managing overlfow-aligment for inherit and initial.

The CSS3 Alignment properties allow pairs as values; that's the case of
overflow-aligment and "legacy" keyword. The StyleBulder should define
custom "initial" and "inherit" handers to properly managing such values.

BUG=249451

Review URL: https://codereview.chromium.org/361113002

git-svn-id: svn://svn.chromium.org/blink/trunk@177384 bbb929c8-8fbe-4397-9dbb-9b2b20218538