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/sslstreamadapter.cc

@@ -82,6 +82,9 @@ bool SSLStreamAdapter::HaveDtlsSrtp() {
 bool SSLStreamAdapter::HaveExporter() {
   return OpenSSLStreamAdapter::HaveExporter();
 }
+bool SSLStreamAdapter::IsBoringSsl() {
+  return OpenSSLStreamAdapter::IsBoringSsl();
+}
 bool SSLStreamAdapter::IsAcceptableCipher(int cipher, KeyType key_type) {
   return OpenSSLStreamAdapter::IsAcceptableCipher(cipher, key_type);
 }