Set mtu for DTLS to 1280

webrtc/base/opensslstreamadapter.cc

Index: webrtc/base/opensslstreamadapter.cc
diff --git a/webrtc/base/opensslstreamadapter.cc b/webrtc/base/opensslstreamadapter.cc
index e12940fbeb90bad6384ed9aa78be7ab2188808ea..51921f143153a171b3573edabd571f265fc9fe22 100644
--- a/webrtc/base/opensslstreamadapter.cc
+++ b/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;
   }