Demote 'line-height' to a low priority property.

Our special handling of the 'line-height' property is no longer
required, since the 'font' shorthand is now expanded in the parser
in all cases (also for system fonts).

R=timloh@chromium.org

Review URL: https://codereview.chromium.org/670373002

git-svn-id: svn://svn.chromium.org/blink/trunk@184629 bbb929c8-8fbe-4397-9dbb-9b2b20218538