Guard socket file descriptor in UDPSocketLibevent

Now UDPSocketLibevent enables file descriptor guard for all UDP sockets
to make sure they are not closed from anywhere else except
UDPSocketLibevent::Close(). This is necessary to debug the linked bug

BUG=461246

Review URL: https://codereview.chromium.org/1053873003

Cr-Commit-Position: refs/heads/master@{#323624}
1 file changed