Add style property for flex-basis

Add the style property for flex-basis. The property should allow all
values acceptable for `width`|`height` with the addition of `content`.

components/style/properties/properties.mako.rs

@@ -4360,6 +4360,9 @@ pub mod longhands {
         }
     </%helpers:longhand>
 
+    ${helpers.predefined_type("flex-basis", "LengthOrPercentageOrAutoOrContent",
+                              "computed::LengthOrPercentageOrAutoOrContent::Auto")}
+
     ${helpers.single_keyword("flex-wrap", "nowrap wrap wrap-reverse", products="gecko")}
 
     // SVG 1.1 (Second Edition)