In order to get the correct computed value of these keywords, we have to
make sure we store the correct computed values in sizing properties in
both inline axis and block axis.
-moz-max-content and -moz-min-content should behave as the property's
initial value in block axis. -moz-fit-content and -moz-available are not
supported in block axis, so we also treat them as initial values.
Differential Revision: https://phabricator.services.mozilla.com/D8290