Set mtu for DTLS to 1280

Set mtu for DTLS to 1280, otherwise it defaults to 256 in recent
versions of openssl.

R=juberti@webrtc.org

Review URL: https://codereview.webrtc.org/1174483002.

Cr-Commit-Position: refs/heads/master@{#9407}

webrtc/base/opensslstreamadapter.cc

@@ -260,6 +260,12 @@ static long stream_ctrl(BIO* b, int cmd, long num, void* ptr) {
       return 0;
     case BIO_CTRL_FLUSH:
       return 1;
+    case BIO_CTRL_DGRAM_QUERY_MTU:
+      // openssl defaults to mtu=256 unless we return something here.
+      // The handshake doesn't actually need to send packets above 1k,
+      // so this seems like a sensible value that should work in most cases.
+      // Webrtc uses the same value for video packets.
+      return 1200;
     default:
       return 0;
   }