style: Make the pres context optional in the style system.

Differential Revision: https://phabricator.services.mozilla.com/D21239

components/style/gecko/wrapper.rs

@@ -1044,9 +1044,13 @@ impl FontMetricsProvider for GeckoFontMetricsProvider {
         device: &Device,
     ) -> FontMetricsQueryResult {
         use crate::gecko_bindings::bindings::Gecko_GetFontMetrics;
+        let pc = match device.pres_context() {
+            Some(pc) => pc,
+            None => return FontMetricsQueryResult::NotAvailable,
+        };
         let gecko_metrics = unsafe {
             Gecko_GetFontMetrics(
-                device.pres_context(),
+                pc,
                 wm.is_vertical() && !wm.is_sideways(),
                 font.gecko(),
                 font_size.0,