Fixes#3356. This makes script elements fetch their source synchronously and execute immediately by default. It also lays some groundwork for future deferred or async script loading.
(Depends on #3716 to fix a content test failure caused by running script during parsing.)
Source-Repo: https://github.com/servo/servo
Source-Revision: 272ae0cc1beb83c839d5fbd746e67f4ae746db99
