GitLab

This is a pre-requisite for http server refactoring,
https://codereview.chromium.org/296053012/.

1) Introduce SocketLibevent for tcp and unix domain sockets.
2) TCPSocketLibevent utilizes SocketLibevent.
3) For backward compatibility, TCPSocketLibevent::AdoptConnectedSocket()
   allows the empty address.

BUG=371906

Review URL: https://codereview.chromium.org/348803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282121 0039d316-1c4b-4281-b951-d872f2087c98