Revisit the lifetime management of embedder shared objects

[paulrouget]

#26793 introduced allocated memory that is released on perform_udpate. It's not a great way to handle object lifetime.

We should have a more elegant way to handle the lifetime of objects shared between servo and the embedder.

See

servo/ports/libsimpleservo/capi/src/lib.rs

Line 542 in 60ee588

prefs::free_prefs();

[paulrouget]

To consider: using the devtools protocol instead of using a low level API.