style: Refactor and add infrastructure for font metrics in style.

This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.

Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.

Then we should make the font provider mandatory, and implement the missing stylo
bits.

components/style/font_metrics.rs

@@ -0,0 +1,35 @@
+/* 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/. */
+
+use Atom;
+use app_units::Au;
+use euclid::Size2D;
+use std::fmt;
+
+/// Represents the font metrics that style needs from a font to compute the
+/// value of certain CSS units like `ex`.
+#[derive(Debug, PartialEq, Clone)]
+pub struct FontMetrics {
+    pub x_height: Au,
+    pub zero_advance_measure: Size2D<Au>,
+}
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum FontMetricsQueryResult {
+    Available(Option<FontMetrics>),
+    NotAvailable,
+}
+
+/// A trait used to represent something capable of providing us font metrics.
+pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
+    /// Obtain the metrics for given font family.
+    ///
+    /// TODO: We could make this take the full list, I guess, and save a few
+    /// virtual calls.
+    ///
+    /// This is not too common in practice though.
+    fn query(&self, _font_name: &Atom) -> FontMetricsQueryResult {
+        FontMetricsQueryResult::NotAvailable
+    }
+}