Add referrer policy pass-through and referrer header logic

add pass-through from doc to http-loader for referrer_policy, ref_URL
add logic for setting referer header
add script pass-through for referrer
add unit tests for setting referer header

components/script/script_thread.rs

@@ -1887,6 +1887,8 @@ impl ScriptThread {
             cors: None,
             pipeline_id: Some(id),
             credentials_flag: true,
+            referrer_policy: load_data.referrer_policy,
+            referrer_url: load_data.referrer_url,
         }, LoadConsumer::Listener(response_target), None)).unwrap();
 
         self.incomplete_loads.borrow_mut().push(incomplete);