Add support for the 'cap' font-relative unit

This is a backport of https://phabricator.services.mozilla.com/D133101,
by Jonathan Kew.

Note that Servo isn't using font metrics yet, so the unit still won't
really work.

components/style/values/generics/calc.rs

@@ -42,6 +42,7 @@ pub enum MinMaxOp {
 pub enum SortKey {
     Number,
     Percentage,
+    Cap,
     Ch,
     Deg,
     Em,