Summary:
I realized that instead of using shared_ptr's type-erasure feature, we can make the EventHandler's destructor virtual and this itself will allow safe deallocation by a pointer to a base class.
We cannot use the same technic for EventTarget thought because having a weak_ptr to this is another feature of shared_ptr that we need.
Reviewed By: mdvacca
Differential Revision: D9775742
fbshipit-source-id: 3c23a163827e8aa9ec731c89ce87051a93afe4ca
e05acf1930