Infrastructure for synchronous script loading

This implements the parts of the "prepare a script element" algorithm that are
required for synchronous scripts.  It also adds some infrastructure for future
support of the `async` and `defer` attributes.
Switch to synchronous script loading

This removes the old code for asyncronously loading scripts during HTML
parsing and then executing them afterward.

Fixes #3356.
Don't force reflow on content changes

This fixes a performance regression caused by the previous patches.  Once we
allowed script and layout to run during parsing, it was running too often
(every time the document changed and called window.reflow).

Fixes #1269.
Make content-tests work with sync script loading
Update wpt metadata for newly passing test