We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
This cannot be done on the Browser or WebDriver ends, or via the
existing run_javascript() IPC endpoint, as we cannot transfer JS objects
through the IPC boundary (yet), only serialized JSON, so the individual
WebDriver steps around script execution need to run in the WebContent
process.