Sandboxed preload scripts are run in the document's script environment

[jgraham]

In https://w3c.github.io/webdriver-bidi/#run-webdriver-bidi-preload-scripts we pass environment settings to evaluate function body, which is wrong in the sandbox case because that environment settings realm is the document's realm, not the sandbox realm.

[jgraham]

#768 has some attempt at fixing this (albeit whilst still leaving key steps in what sandboxing means undefined).