Add read support of RtpStreamId/RepairedRtpStreamId header extensions.

BUG=webrtc:7433

Review-Url: https://codereview.webrtc.org/2805023002
Cr-Commit-Position: refs/heads/master@{#17759}

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -77,6 +77,8 @@ enum RTPExtensionType {
   kRtpExtensionTransportSequenceNumber,
   kRtpExtensionPlayoutDelay,
   kRtpExtensionVideoContentType,
+  kRtpExtensionRtpStreamId,
+  kRtpExtensionRepairedRtpStreamId,
   kRtpExtensionNumberOfExtensions  // Must be the last entity in the enum.
 };