Pass datachannel priority in DC open messages

This adds priority to the API configuration of datachannels,
and passes the value in the OPEN message.

It does not yet influence SCTP prioritization of messages.

Bug: chromium:1083227
Change-Id: I46ddd1eefa0e3d07c959383788b9e80fcbfa38d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175107
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Taylor <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31287}

api/priority.h

@@ -0,0 +1,26 @@
+/*
+ *  Copyright 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_PRIORITY_H_
+#define API_PRIORITY_H_
+
+namespace webrtc {
+
+// GENERATED_JAVA_ENUM_PACKAGE: org.webrtc
+enum class Priority {
+  kVeryLow,
+  kLow,
+  kMedium,
+  kHigh,
+};
+
+}  // namespace webrtc
+
+#endif  // API_PRIORITY_H_