Bug 1509931 - Add support for chrome-only environment variables. r=stransky

This bit is taken straight from D73454 (I reviewed it but I guess
another pair of eyes is ok, it's really straight-forward).

Co-authored-by: Nicklas Boman <smurfd@gmail.com>

Differential Revision: https://phabricator.services.mozilla.com/D128679

servo/components/style/custom_properties.rs

@@ -71,10 +71,19 @@ static ENVIRONMENT_VARIABLES: [EnvironmentVariable; 4] = [
     make_variable!(atom!("safe-area-inset-right"), get_safearea_inset_right),
 ];
 
+static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 0] = [
+];
+
 impl CssEnvironment {
     #[inline]
     fn get(&self, name: &Atom, device: &Device) -> Option<VariableValue> {
-        let var = ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name)?;
+        if let Some(var) = ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name) {
+            return Some((var.evaluator)(device));
+        }
+        if !device.is_chrome_document() {
+            return None;
+        }
+        let var = CHROME_ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name)?;
         Some((var.evaluator)(device))
     }
 }

servo/components/style/gecko/media_queries.rs

@@ -367,4 +367,10 @@ impl Device {
             bindings::Gecko_IsSupportedImageMimeType(mime_type.as_ptr(), mime_type.len() as u32)
         }
     }
+
+    /// Return whether the document is a chrome document.
+    #[inline]
+    pub fn is_chrome_document(&self) -> bool {
+        self.pref_sheet_prefs().mIsChrome
+    }
 }