This commit introduces a new preference option
mathml.mathsize_names.disabled to disable mathsize keyword values. For
now, these are only disabled in Nightly builds.
* test_bug553917.html is updated to check that these values now cause an error
message to be logged into the console.
* The old test 355548-3.xml checks support for mathsize names and also uses
several features that are going to be deprecated. So it is just run with
the proper preference adjustment.
* mathml/relations/css-styling/mathsize-attribute-legacy-values.html passes
after this change so the failure expectation is removed.
* mathml/relations/css-styling/mathsize-attribute-css-keywords.html is added
to check that CSS keywords won't be supported when we switch to using the
CSS parser in the future. This test passes now that we no longer accept
the "small" keyword.