Support prefixed intrinsic size value for flex-basis.

auto() and zero() are used in the parser for flex shorthand property.

components/style/properties/helpers.mako.rs

@@ -1090,6 +1090,23 @@
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct SpecifiedValue(pub ${length_type});
 
+        % if length_type == "MozLength":
+        impl SpecifiedValue {
+            /// Returns the `auto` value.
+            pub fn auto() -> Self {
+                use values::specified::length::LengthOrPercentageOrAuto;
+                SpecifiedValue(MozLength::LengthOrPercentageOrAuto(LengthOrPercentageOrAuto::Auto))
+            }
+
+            /// Returns a value representing a `0` length.
+            pub fn zero() -> Self {
+                use values::specified::length::{LengthOrPercentageOrAuto, NoCalcLength};
+                SpecifiedValue(MozLength::LengthOrPercentageOrAuto(
+                    LengthOrPercentageOrAuto::Length(NoCalcLength::zero())))
+            }
+        }
+        % endif
+
         #[inline]
         pub fn get_initial_value() -> computed_value::T {
             use values::computed::${length_type};

components/style/properties/longhand/position.mako.rs

@@ -134,18 +134,23 @@ ${helpers.predefined_type("order", "Integer", "0",
                           animation_value_type="ComputedValue",
                           spec="https://drafts.csswg.org/css-flexbox/#order-property")}
 
-// FIXME: Gecko doesn't support content value yet.
-// FIXME: This property should be animatable.
-${helpers.predefined_type("flex-basis",
-                          "LengthOrPercentageOrAuto" if product == "gecko" else
-                          "LengthOrPercentageOrAutoOrContent",
-                          "computed::LengthOrPercentageOrAuto::Auto" if product == "gecko" else
-                          "computed::LengthOrPercentageOrAutoOrContent::Auto",
-                          "parse_non_negative",
-                          spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
-                          extra_prefixes="webkit",
-                          animation_value_type="ComputedValue" if product == "gecko" else "none")}
-
+% if product == "gecko":
+    // FIXME: Gecko doesn't support content value yet.
+    ${helpers.gecko_size_type("flex-basis", "MozLength", "auto()",
+                              logical=False,
+                              spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
+                              extra_prefixes="webkit",
+                              animation_value_type="ComputedValue")}
+% else:
+    // FIXME: This property should be animatable.
+    ${helpers.predefined_type("flex-basis",
+                              "LengthOrPercentageOrAutoOrContent",
+                              "computed::LengthOrPercentageOrAutoOrContent::Auto",
+                              "parse_non_negative",
+                              spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
+                              extra_prefixes="webkit",
+                              animation_value_type="none")}
+% endif
 % for (size, logical) in ALL_SIZES:
     <%
       spec = "https://drafts.csswg.org/css-box/#propdef-%s"

components/style/properties/shorthand/position.mako.rs

@@ -78,7 +78,7 @@
                 }
             }
             if basis.is_none() {
-                if let Ok(value) = input.try(|input| longhands::flex_basis::parse(context, input)) {
+                if let Ok(value) = input.try(|input| longhands::flex_basis::parse_specified(context, input)) {
                     basis = Some(value);
                     continue
                 }