insertable streams: add rtp sequence number to incoming audio metadata

spec change w3c/webrtc-encoded-transform#154

drive-by:
- fix idl of payloadType to be octet instead of short
- remove unused contributing_sources vector on audio frame

BUG=TBD

Change-Id: Ic8dc8dcebb035a8f79823a8ca70e02ac1d6788cf

webrtc-encoded-transform/RTCPeerConnection-insertable-streams-audio.https.html

@@ -56,6 +56,7 @@
             // frames are not passed correctly between sender and receiver.
             assert_unreached("Incorrect frame received");
           }
+          assert_not_equals(result.value.getMetadata().sequenceNumber, undefined);
 
           if (numVerifiedFrames == numFramesToSend)
             resolve();