Preserve sourceURL comment on style sheets

In addition to the sourceMappingURL comment, there is a second special
comment, "sourceURL", that can be used to set the "display name" of a
style sheet for developer tools.  This name is also used as the base
URL for the source-map URL resolution algorithm.  sourceURL is
described here:
https://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/
The devtools feature bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=880831

This patch changes servo to preserve and expose this value for use in M-C.

components/script/dom/htmlmetaelement.rs

@@ -110,6 +110,7 @@ impl HTMLMetaElement {
                             quirks_mode: document.quirks_mode(),
                             url_data: RwLock::new(window_from_node(self).get_url()),
                             source_map_url: RwLock::new(None),
+                            source_url: RwLock::new(None),
                         },
                         media: Arc::new(shared_lock.wrap(MediaList::empty())),
                         shared_lock: shared_lock.clone(),