My understanding of what goes wrong is that

• StreamList has a raw pointer to a CacheStreamControlParent (mStreamControl)
• StreamList calling mStreamControl->CloseAll(); can indirectly re-enter a StreamList method for the same calling instance and delete its mStreamControl, which means it's deleting itself
• Before CloseAll returns and after the mStreamControl is deleted, it (mStreamControl) would call one of its own methods, which is a UAF

Removing StreamList's member pointer prevents the mStreamControl from indirectly deleting itself. Shutdown is just additional cleanup to delete (what used to be) mStreamControl