Bug 1837570 - Canonical::ConnectMirror must also sync its value.

This is a fixup after a bad adaptation of a review comment on D180427 regressed
the value syncing on connect.

Differential Revision: https://phabricator.services.mozilla.com/D181811

xpcom/threads/StateMirroring.h

@@ -144,8 +144,8 @@ class Canonical {
       MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
       MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aMirror->OwnerThread()),
                  "Can't get coherency without tail dispatch");
-      mMirrors.AppendElement(aMirror);
       aMirror->ConnectedOnCanonicalThread(this);
+      AddMirror(aMirror);
     }
 
     void AddMirror(AbstractMirror<T>* aMirror) override {