Auto merge of #7825 - eefriedman:div-align, r=mbrubeck

Fully implement the "align descendants" rule for div.

This adds -servo-left and -servo-right to complement -servo-center.

~~This intentionally doesn't try to address issue #7301.~~  Commit added to address #7301.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7825)
<!-- Reviewable:end -->

components/style/properties.mako.rs

@@ -2012,6 +2012,8 @@ pub mod longhands {
                 center("center") => 4,
                 justify("justify") => 5,
                 servo_center("-servo-center") => 6,
+                servo_left("-servo-left") => 7,
+                servo_right("-servo-right") => 8,
             }
         }
         #[inline] pub fn get_initial_value() -> computed_value::T {