RtpTransceiverInterface: add header_extensions_to_offer()

This change adds exposure of a new transceiver method for getting
the total set of supported extensions stored as an attribute,
and their direction. If the direction is kStopped, the extension
is not signalled in Unified Plan SDP negotiation.

Note: SDP negotiation is not modified by this change.

Changes:
- RtpHeaderExtensionCapability gets a new RtpTransceiverDirection,
  indicating either kStopped (extension available but not signalled),
  or other (extension signalled).
- RtpTransceiver gets the new method as described above. The
  default value of the attribute comes from the voice and video
  engines as before.

https://chromestatus.com/feature/5680189201711104.
go/rtp-header-extension-ip
Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/65YdUi02yZk

Bug: chromium:1051821
Change-Id: I440443b474db5b1cfe8c6b25b6c10a3ff9c21a8c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170235
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30800}

api/rtp_transceiver_interface.h

@@ -20,21 +20,13 @@
 #include "api/rtp_parameters.h"
 #include "api/rtp_receiver_interface.h"
 #include "api/rtp_sender_interface.h"
+#include "api/rtp_transceiver_direction.h"
 #include "api/scoped_refptr.h"
 #include "rtc_base/ref_count.h"
 #include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
-// https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiverdirection
-enum class RtpTransceiverDirection {
-  kSendRecv,
-  kSendOnly,
-  kRecvOnly,
-  kInactive,
-  kStopped,
-};
-
 // Structure for initializing an RtpTransceiver in a call to
 // PeerConnectionInterface::AddTransceiver.
 // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiverinit
@@ -134,6 +126,13 @@ class RTC_EXPORT RtpTransceiverInterface : public rtc::RefCountInterface {
       rtc::ArrayView<RtpCodecCapability> codecs);
   virtual std::vector<RtpCodecCapability> codec_preferences() const;
 
+  // Readonly attribute which contains the set of header extensions that was set
+  // with SetOfferedRtpHeaderExtensions, or a default set if it has not been
+  // called.
+  // https://w3c.github.io/webrtc-extensions/#rtcrtptransceiver-interface
+  virtual std::vector<RtpHeaderExtensionCapability> HeaderExtensionsToOffer()
+      const;
+
  protected:
   ~RtpTransceiverInterface() override = default;
 };