libservo: Move WebDriver messages to the embedder crate (#35602)

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>

Cargo.lock

@@ -1852,6 +1852,7 @@ version = "0.0.1"
 dependencies = [
  "base",
  "cfg-if",
+ "cookie 0.18.1",
  "crossbeam-channel",
  "euclid",
  "http 1.2.0",
@@ -1862,10 +1863,13 @@ dependencies = [
  "malloc_size_of_derive",
  "num-derive",
  "num-traits",
+ "pixels",
  "serde",
  "servo_malloc_size_of",
  "servo_url",
+ "style_traits",
  "url",
+ "webdriver",
  "webrender_api",
  "webxr-api",
 ]