style: Add support for chrome-only environment variables

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

components/style/gecko/media_queries.rs

@@ -439,4 +439,10 @@ 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
+    }
 }