Taking ownership by plain pointer in LargeIconResult a bit ugly. Do
 - change these FallbackIconStyle constructors to take unique_ptr, and
 - change the callbacks to pass unique_ptrs of LargeIconResult

This would allow us to get rid of "new ..." and to use std::move out things out LargeIconResult if you need to.
(see, e.g. LargeIconCache::CloneLargeIconResult)