This is so the tests needn't be sensitive to the clock. It is, unfortunately, a
test-only hook, but the DTLS retransmit/timeout logic more-or-less requires it
currently. Use this hook to, for now, freeze the clock at zero. This makes the
tests deterministic.
It might be worth designing a saner API in the future. The current one,
notably, requires that the caller's clock be compatible with the one we
internally use. It's also not clear whether the caller needs to call
DTLSv1_handle_timeout or can just rely on the state machine doing it internally
(as it does do). But mock clocks are relatively tame and WebRTC wants to
compile against upstream OpenSSL for now, so we're limited in how much new API
we can build.
Change-Id: I7aad51570596f69275ed0fc1a8892393e4b7ba13
Reviewed-on: https://boringssl-review.googlesource.com/3210
Reviewed-by: Adam Langley <agl@google.com>
377fc3160c