Basic handling framework for presentation attributes in Stylo, with handling for font-size and color

This introduces a basic framework for servo's style system to be able
to query the style of presentation attributes which it can then insert
into the cascade. It uses that framework to implement the size and
color attributes on <font>.

There are a number of improvements that can be done on top of this:

 - Implement all other properties
 - Abstractify the ruledata parameter of the mappers using templates or virtual dispatch so that it can be a Servo decl block instead
 - Implement aforementiond abstraction over Servo decl blocks (this obsoletes the code in the first item above, so it might just be better to skip that and directly do this)
 - Replace uses of nsHTMLStyleSheet with an abstract base class containing common elements between Servo and Gecko

I'd prefer for these to be done in separate steps.

components/style/values/specified/length.rs

@@ -293,6 +293,20 @@ impl NoCalcLength {
         })
     }
 
+    /// https://drafts.csswg.org/css-fonts-3/#font-size-prop
+    pub fn from_font_size_int(i: u8) -> Self {
+        let au = match i {
+            0 | 1 => Au::from_px(FONT_MEDIUM_PX) * 3 / 4,
+            2 => Au::from_px(FONT_MEDIUM_PX) * 8 / 9,
+            3 => Au::from_px(FONT_MEDIUM_PX),
+            4 => Au::from_px(FONT_MEDIUM_PX) * 6 / 5,
+            5 => Au::from_px(FONT_MEDIUM_PX) * 3 / 2,
+            6 => Au::from_px(FONT_MEDIUM_PX) * 2,
+            _ => Au::from_px(FONT_MEDIUM_PX) * 3,
+        };
+        NoCalcLength::Absolute(au)
+    }
+
     /// Parse a given absolute or relative dimension.
     pub fn parse_dimension(value: CSSFloat, unit: &str) -> Result<NoCalcLength, ()> {
         match_ignore_ascii_case! { unit,
@@ -429,6 +443,11 @@ impl Length {
         NoCalcLength::parse_dimension(value, unit).map(Length::NoCalc)
     }
 
+    /// https://drafts.csswg.org/css-fonts-3/#font-size-prop
+    pub fn from_font_size_int(i: u8) -> Self {
+        Length::NoCalc(NoCalcLength::from_font_size_int(i))
+    }
+
     #[inline]
     fn parse_internal(input: &mut Parser, context: AllowedNumericType) -> Result<Length, ()> {
         match try!(input.next()) {