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/specified/calc.rs

@@ -168,6 +168,7 @@ impl generic::CalcNodeLeaf for Leaf {
                     FontRelativeLength::Ch(..) => SortKey::Ch,
                     FontRelativeLength::Em(..) => SortKey::Em,
                     FontRelativeLength::Ex(..) => SortKey::Ex,
+                    FontRelativeLength::Cap(..) => SortKey::Cap,
                     FontRelativeLength::Rem(..) => SortKey::Rem,
                 },
                 NoCalcLength::ViewportPercentage(ref vp) => match *vp {