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/data_channel_interface.h

@@ -20,6 +20,7 @@
 #include <string>
 
 #include "absl/types/optional.h"
+#include "api/priority.h"
 #include "api/rtc_error.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/copy_on_write_buffer.h"
@@ -61,6 +62,9 @@ struct DataChannelInit {
 
   // The stream id, or SID, for SCTP data channels. -1 if unset (see above).
   int id = -1;
+
+  // https://w3c.github.io/webrtc-priority/#new-rtcdatachannelinit-member
+  absl::optional<Priority> priority;
 };
 
 // At the JavaScript level, data can be passed in as a string or a blob, so
@@ -154,6 +158,7 @@ class RTC_EXPORT DataChannelInterface : public rtc::RefCountInterface {
   // If negotiated in-band, this ID will be populated once the DTLS role is
   // determined, and until then this will return -1.
   virtual int id() const = 0;
+  virtual Priority priority() const { return Priority::kLow; }
   virtual DataState state() const = 0;
   // When state is kClosed, and the DataChannel was not closed using
   // the closing procedure, returns the error information about the closing.