Enable stretch keyword (#81)

In #79 we forgot to set the relevant pref to true.
We will be adding support for these sizing keywords in Servo, so better
enable it so that we can test that it works well.

Signed-off-by: Oriol Brufau <[email protected]>

Author: Loirooriol

style_static_prefs/src/lib.rs

@@ -24,6 +24,9 @@ macro_rules! pref {
     ("layout.css.basic-shape-xywh.enabled") => {
         true
     };
+    ("layout.css.stretch-size-keyword.enabled") => {
+        true
+    };
     ($string:literal) => {
         false
     };