Support flexbox shorthand properties

Support the `flex` and `flex-flow` shorthand properties in
servo. Currently they are marked as experimental, so they are added to
'__dir__.ini'.

Thanks SimonSapin and jdm for help :)

tests/wpt/metadata-css/css-flexbox-1_dev/__dir__.ini

@@ -1,4 +1,5 @@
 prefs: ["layout.flex.enabled:true",
+        "layout.flex-flow.enabled:true",
         "layout.flex-direction.enabled:true",
         "layout.flex-wrap.enabled:true",
         "layout.flex-grow.enabled:true",