It does not represent `<length> | <percentage>`, but `<length-percentage>`, so
`LengthOrPercentage` is not the right name.

This patch is totally autogenerated using:

rg 'LengthOrPercentage' servo | cut -d : -f 1 | sort | uniq > files
for file in $(cat files); do sed -i "s#LengthOrPercentage#LengthPercentage#g" $file; done

Differential Revision: https://phabricator.services.mozilla.com/D15812