style: Add some flags to the computed values.

This will allow us to fix propagation of text-decoration, and also to implement
inlinization of ruby kids.

Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/properties/computed_value_flags.rs

@@ -0,0 +1,45 @@
+/* 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/. */
+
+//! Misc information about a given computed style.
+
+use properties::{ComputedValues, StyleBuilder};
+
+bitflags! {
+    /// Misc information about a given computed style.
+    pub flags ComputedValueFlags: u8 {
+        /// Whether the style or any of the ancestors has a text-decoration
+        /// property that should get propagated to descendants.
+        ///
+        /// text-decoration is a reset property, but gets propagated in the
+        /// frame/box tree.
+        const HAS_TEXT_DECORATION_LINE = 1 << 0,
+    }
+}
+
+impl ComputedValueFlags {
+    /// Get the computed value flags for the initial style.
+    pub fn initial() -> Self {
+        Self::empty()
+    }
+
+    /// Compute the flags for this style, given the parent style.
+    pub fn compute(
+        this_style: &StyleBuilder,
+        parent_style: &ComputedValues,
+    ) -> Self {
+        let mut ret = Self::empty();
+
+        // FIXME(emilio): This feels like it wants to look at the
+        // layout_parent_style, but the way it works in Gecko means it's not
+        // needed (we'd recascade a bit more when it changes, but that's fine),
+        // and this way it simplifies the code for text styles and similar.
+        if parent_style.flags.contains(HAS_TEXT_DECORATION_LINE) ||
+           !this_style.get_text().clone_text_decoration_line().is_empty() {
+            ret.insert(HAS_TEXT_DECORATION_LINE);
+        }
+
+        ret
+    }
+}