Usually the view will accept any provider, but it should be possible to create a view that returns only matches from a specific list of providers, for example an experiment may want to only query its own providers, or replace a system provider. This could be done by add a providerWhitelist to the QueryContext, and having the ProvidersManager join this list with the user's one.

Pushed by mak77@bonardo.net: https://hg.mozilla.org/integration/autoland/rev/c00b8c0f29ba Allow the view to limit searches to specific providers. r=adw