Summary:
Even though MockAsyncSSLSocket's initializes AsyncSSLSocket, which should
initialize AsyncSocket, this does not actually happen because AsyncSSLSocket
virtually inherits from AsyncSocket.

Because of this, prior to this diff, the MockAsyncSSLSocket was not properly
setting its EventBase.

Reviewed By: knekritz

Differential Revision: D5676596

fbshipit-source-id: 5f3c0e848179cb5eb4d2dc4921a11e7c04d7c0e0