layout: Implement text-indent per CSS 2.1 § 16.1.

I had to use a somewhat unconventional method of computing text
indentation (propagating from blocks down to inlines) because of the way
containing blocks are handled in Servo.

(As a side note, neither Gecko nor WebKit correctly handles percentages
in `text-align`, at least incrementally -- i.e. when the percentages are
relative to the viewport and the viewport is resized.)

components/style/properties/mod.rs.mako

@@ -1111,6 +1111,8 @@ pub mod longhands {
         }
     </%self:single_component_value>
 
+    ${predefined_type("text-indent", "LengthOrPercentage", "computed::LP_Length(Au(0))")}
+
     ${new_style_struct("Text", is_inherited=False)}
 
     <%self:longhand name="text-decoration">