This introduces a new type to ContentParent which acts as a weak handle to the actor and is safe to hold and manipulate from any thread. This replaces accesses of the `ContentParent` type from the background thread, as they were error-prone due to ContentParent not being threadsafe-refcounted. The bulk of this patch is piping the new type through to the places it is required, and removing now-unecessary extra complexity. Differential Revision: https://phabricator.services.mozilla.com/D162346