This is the first step toward moving the WebDriver implementation to
servoshell. This move will make it possible to start testing the
embedding API with WebDriver. See [this zulip thread][a] for more details.
While WebDriver will be able to use a lot of API commands to do what it
is doing now, there will still need to be some "cheat codes" for more
gnarly access to `ScriptThread` details. That's why we likely won't be
able to remove all WebDriver-specific messages from the API -- but maybe
they will be useful for embedders somehow.
A couple messages have to change as they depended on `script_traits`
types, particularly those that used `WindowSizeData` and `LoadData`. I
think this helps to encapsulate the WebDriver commands a bit more
though.
[a]: https://servo.zulipchat.com/#narrow/channel/437943-embedding/topic/webdriver.20as.20embedding.20api.20playgound
Signed-off-by: Martin Robinson <mrobinson@igalia.com>