Add a configuration parameter for using the media transport for data channels.

Adds a field |use_media_transport_for_data_channels| to RTCConfiguration.
PeerConnection requires a media transport factory to be set if this bit
is set.  As with |use_media_transport|, the value may not be modified
after setting the local or remote description.

If either |use_media_transport| or |use_media_transport_for_data_channel| is
set, PeerConnection uses its media transport factory when creating a JSEP
transport controller.

PeerConnection stops unconditionally using media transport in
CreateVoiceChannel, as it may be present only for use in data channels.  It uses
the media transport if it is present and |use_media_transport| is set.

Bug: webrtc:9719
Change-Id: I59d4ce8f7531fd19d9c17eefe033f063f663ebcc
Reviewed-on: https://webrtc-review.googlesource.com/c/109041
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Peter Slatala <psla@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25507}

sdk/objc/api/peerconnection/RTCConfiguration.h

@@ -188,6 +188,12 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, assign) BOOL useMediaTransport;
 
+/**
+ * If MediaTransportFactory is provided in PeerConnectionFactory, this flag informs PeerConnection
+ * that it should use the MediaTransportInterface for data channels.
+ */
+@property(nonatomic, assign) BOOL useMediaTransportForDataChannels;
+
 /**
  * Defines advanced optional cryptographic settings related to SRTP and
  * frame encryption for native WebRTC. Setting this will overwrite any