Define WebDriver endpoints for freeze and resume

[foolip]

This is to track step 2 of #19, which appears to be the only remaining step.

These endpoints can be define in the Page Lifecycle spec directly, with https://w3c.github.io/permissions/#automation being an example of how that's done.

The endpoints in ChromeDriver are:

• POST /session/{session id}/goog/page/freeze
• POST /session/{session id}/goog/page/resume

The remote end steps should probably use one of the algorithms in https://wicg.github.io/page-lifecycle/#changing-frozenness.

[stephenmcgruer]

cc @Hexcles, since we chatted about this earlier