This commit splits the `GeckoViewContent` actor in two parts:

- `GeckoViewContent` proper, which runs unconditionally and handles code that
  needs to run regardless of whether we have a delegate installed or not.

- `ContentDelegate` which runs only when a delegate is first installed.

This emulates the previous paradigm of installing some listeners only when the
delegate is installed.

I discussed it briefly with :nika and she thinks that splitting modules in two
should not affect performance in a measurable manner.

Note that actors cannot be registered per-window, so we will get messages from
all windows as long as one content delegate is registered.

Differential Revision: https://phabricator.services.mozilla.com/D86776