Similarly to https://bugzilla.mozilla.org/show_bug.cgi?id=1355890,
we want to show a visual UX cue when the remote agent is active,
to indicate to the user that the browser session exposed to remote
control.

browser/base/content/browser.js is not passed through the preprocessor
which means we can’t use #ifdef ENABLE_REMOTE_AGENT. Once the
remote agent is enabled in Nightly by default, we can use
AppConstants.NIGHTLY_BUILD to guard the import of RemoteAgent.jsm.

Attaching a WIP patch that can be applied once bug 1533831 has been
fixed.

I originally wanted to recreate the nsIRemoteAgent XPCOM interface
that we had in the original prototype, because if we want to move
to a Rust-based HTTPD in the future we would need that.

However, since browser.js now allows for ChromeUtils imports and
we have RemoteAgent.jsm doing the right thing, we can make do with
just importing it for the time being. This seems like the least
invasive approach.

Besides that, the patch reuses the visual cue I implemented for
Marionette years ago. That could do with a stylistic update, but
serves the purpose without introducing any extra code.

Not planning to do any more work on this for the time being.

This has become a priority because of the sec-review.

I think we need to wait for bug 1543115 because it makes substantial
changes to remote/RemoteAgent.jsm, and also includes the system
observer notification we need.

This changes the event Marionette emits when it is running from
"remote-active" to "remote-listening".

The background for this change is that the forthcoming Chromium-based
remote protocol uses the same observer notification to indicate
when it listens for browser-external connections. This means that
with this change, the visual hint presented to the user will also
be applied when the remote agent starts its HTTPD listener.

Unlike Marionette however, the remote debugging protocol may be used
for browser-internal applications and it is not appropriate to signal
that the browser is under remote control under those circumstances,
hence the naming change away from "active".

We want to display a visual hint to the user that the browser session
is under control by a browser-external client when the remote agent
is listening. The hint is the same as for when Marionette is active.

https://hg.mozilla.org/mozilla-central/rev/3ea0212843da
https://hg.mozilla.org/mozilla-central/rev/805533fa08a8