Fix inheritance of baseline-shift <length> values

Needs custom_inherit because of the enum+length combination.

BUG=304006

Review URL: https://codereview.chromium.org/330913005

git-svn-id: svn://svn.chromium.org/blink/trunk@176928 bbb929c8-8fbe-4397-9dbb-9b2b20218538