Revert "remove mslabel and mslabel ssrc-specific attributes"

This reverts commit e3f257c4ee2079dee14ec8425eec691db3a9757c.

Reason for revert: Breaks downstream projects

Original change's description:
> remove mslabel and mslabel ssrc-specific attributes
> 
> Removes support for parsing and serializing
>   a=ssrc:1 mslabel:stream
>   a=ssrc:1 label:track
> which have been superceeded by
>   a=ssrc:1 msid:stream track
> a long time ago.
> 
> Bug: webrtc:7110
> Change-Id: I3aca47728098b6e7e049b82ed34c59426d411c41
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168244
> Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30801}

TBR=kthelgason@webrtc.org,hta@webrtc.org,philipp.hancke@googlemail.com

Change-Id: Ibd0ad11d2dee9f54bacab3dcca61dedccfc2c120
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:7110
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170620
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30805}
diff --git a/sdk/objc/unittests/RTCSessionDescriptionTest.mm b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
index 6acfad9..0807eed 100644
--- a/sdk/objc/unittests/RTCSessionDescriptionTest.mm
+++ b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
@@ -90,6 +90,8 @@
           "a=maxptime:60\r\n"
           "a=ssrc:1504474588 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:1504474588 msid:ARDAMS ARDAMSa0\r\n"
+          "a=ssrc:1504474588 mslabel:ARDAMS\r\n"
+          "a=ssrc:1504474588 label:ARDAMSa0\r\n"
           "m=video 9 UDP/TLS/RTP/SAVPF 100 116 117 96\r\n"
           "c=IN IP4 0.0.0.0\r\n"
           "a=rtcp:9 IN IP4 0.0.0.0\r\n"
@@ -118,8 +120,12 @@
           "a=ssrc-group:FID 498297514 1644357692\r\n"
           "a=ssrc:498297514 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:498297514 msid:ARDAMS ARDAMSv0\r\n"
+          "a=ssrc:498297514 mslabel:ARDAMS\r\n"
+          "a=ssrc:498297514 label:ARDAMSv0\r\n"
           "a=ssrc:1644357692 cname:V+FdIC5AJpxLhdYQ\r\n"
-          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n";
+          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n"
+          "a=ssrc:1644357692 mslabel:ARDAMS\r\n"
+          "a=ssrc:1644357692 label:ARDAMSv0\r\n";
 }
 
 @end