Change initial DTLS retransmission timer from 1 second to 50ms.

This will help ensure a timely DTLS handshake when there's packet
loss. It will likely result in spurious retransmissions (since the
RTT is usually > 50ms), but since exponential backoff is still used,
there will at most be ~4 extra retransmissions. For a time-sensitive
application like WebRTC this seems like a reasonable tradeoff.

R=pthatcher@webrtc.org, juberti@chromium.org, juberti@webrtc.org

Review URL: https://codereview.webrtc.org/1981463002 .

Committed: https://crrev.com/1e435628366fb9fed71632369f05928ed857d8ef
Cr-Original-Commit-Position: refs/heads/master@{#12853}
Cr-Commit-Position: refs/heads/master@{#13159}

webrtc/base/timeutils.h

@@ -53,6 +53,11 @@ class ClockInterface {
 // that uses it, eliminating the need for a global variable and this function.
 ClockInterface* SetClockForTesting(ClockInterface* clock);
 
+// Returns the actual system time, even if a clock is set for testing.
+// Useful for timeouts while using a test clock, or for logging.
+uint64_t SystemTimeNanos();
+int64_t SystemTimeMillis();
+
 // Returns the current time in milliseconds in 32 bits.
 uint32_t Time32();