Add some way of accessing warnings/DOM errors/console.log output on embedded devices

[jdm]

For app developers who are not developing Servo, they won't have the VS output window to show them useful page debugging info. We need some mechanism to allow getting this data from the device. Possibly we can run an HTTP server?

[asajeffrey]

Is this going to be specific to the device? E.g. on magicleap there's custom code in the embedder to hook up Rust logging to ML_LOG.

[jdm]

Probably? Maybe we'll want a general embedding API for it.

[jdm]

We can make use of the existing devtools server while still works in FF 68.