style: Get rid of GetParentAllowServo in implementation of CSS 'justify-items' property.

Bug: 1384542
Reviewed-by: heycam
MozReview-Commit-ID: 4qydjqSoVXs

components/style/values/computed/align.rs

@@ -0,0 +1,75 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Values for CSS Box Alignment properties
+//!
+//! https://drafts.csswg.org/css-align/
+
+use std::fmt;
+use style_traits::ToCss;
+use values::computed::{ComputedValueAsSpecified, Context, ToComputedValue};
+use values::specified;
+
+pub use super::specified::{AlignItems, AlignJustifyContent, AlignJustifySelf};
+
+/// The computed value for the `justify-items` property.
+///
+/// Need to carry around both the specified and computed value to handle the
+/// special legacy keyword. Sigh.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct JustifyItems {
+    /// The specified value for the property. Can contain `auto`.
+    pub specified: specified::JustifyItems,
+    /// The computed value for the property. Cannot contain `auto`.
+    pub computed: specified::JustifyItems,
+}
+
+impl ToCss for JustifyItems {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        self.computed.to_css(dest)
+    }
+}
+
+impl JustifyItems {
+    /// Returns the `auto` value.
+    pub fn auto() -> Self {
+        Self {
+            specified: specified::JustifyItems::auto(),
+            computed: specified::JustifyItems::normal(),
+        }
+    }
+}
+
+impl ToComputedValue for specified::JustifyItems {
+    type ComputedValue = JustifyItems;
+
+    /// https://drafts.csswg.org/css-align/#valdef-justify-items-legacy
+    fn to_computed_value(&self, _context: &Context) -> JustifyItems {
+        use values::specified::align;
+        let specified = *self;
+        let computed =
+            if self.0 != align::ALIGN_AUTO {
+                *self
+            } else {
+                // If the inherited value of `justify-items` includes the
+                // `legacy` keyword, `auto` computes to the inherited value,
+                // but we assume it computes to `normal`, and handle that
+                // special-case in StyleAdjuster.
+                Self::normal()
+            };
+
+        JustifyItems { specified, computed }
+    }
+
+    #[inline]
+    fn from_computed_value(computed: &JustifyItems) -> Self {
+        computed.specified
+    }
+}
+
+impl ComputedValueAsSpecified for AlignItems {}
+impl ComputedValueAsSpecified for AlignJustifyContent {}
+impl ComputedValueAsSpecified for AlignJustifySelf {}