Access browsing context without panic

Current browsing context accessor in Document unwrap the browsing context option which prevents document to handle correctly the case when there is no browsing context.

components/script/dom/document.rs

@@ -390,7 +390,7 @@ impl Document {
     #[inline]
     pub fn browsing_context(&self) -> Option<Root<BrowsingContext>> {
         if self.has_browsing_context {
-            Some(self.window.browsing_context())
+            self.window.maybe_browsing_context()
         } else {
             None
         }