style: Refactor and add infrastructure for font metrics in style.

This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.

Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.

Then we should make the font provider mandatory, and implement the missing stylo
bits.

components/style/properties/helpers/animated_properties.mako.rs

@@ -418,7 +418,7 @@ impl Interpolate for CalcLengthOrPercentage {
         }
 
         Ok(CalcLengthOrPercentage {
-            length: try!(interpolate_half(self.length, other.length, progress)),
+            length: try!(self.length.interpolate(&other.length, progress)),
             percentage: try!(interpolate_half(self.percentage, other.percentage, progress)),
         })
     }