Bug 1584007 - FutureClientSourceParent should keep ClientManagerServi…

…ce alive r=asuth

The strong reference will be dropped when FutureClientSourceParent is removed,
and we guarantee that all FutureClientSourceParents will either be removed or
replaced with a ClientSourceParent (or at least this is a guarantee that's
supposed to be true).

Depends on D66529

Differential Revision: https://phabricator.services.mozilla.com/D68041

--HG--
extra : moz-landing-system : lando

dom/clients/manager/ClientManagerService.h

@@ -64,6 +64,7 @@ class ClientManagerService final {
    private:
     const mozilla::ipc::PrincipalInfo mPrincipalInfo;
     MozPromiseHolder<SourcePromise> mPromiseHolder;
+    RefPtr<ClientManagerService> mService = ClientManagerService::GetInstance();
   };
 
   using SourceTableEntry =