Reland of Enable audio streams to send padding. (patchset #4 id:60001 of https://codereview.webrtc.org/2652893004/ )

Original issue's description:
> Enable audio streams to send padding.
>
> Useful if bitrate probing is to be used with audio streams.
>
> BUG=webrtc:7043
>
> Review-Url: https://codereview.webrtc.org/2652893004
> Cr-Commit-Position: refs/heads/master@{#16404}
> Committed: https://chromium.googlesource.com/external/webrtc/+/e35f89a484ca376d5c187d166714eba578dfadc3

BUG=webrtc:7043

Review-Url: https://codereview.webrtc.org/2675703002
Cr-Commit-Position: refs/heads/master@{#16433}
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index e0bcebd..d1f70ee 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -617,6 +617,15 @@
   return rtp_sender_.DeregisterRtpHeaderExtension(type);
 }
 
+bool ModuleRtpRtcpImpl::HasBweExtensions() const {
+  return rtp_sender_.IsRtpHeaderExtensionRegistered(
+             kRtpExtensionTransportSequenceNumber) ||
+         rtp_sender_.IsRtpHeaderExtensionRegistered(
+             kRtpExtensionAbsoluteSendTime) ||
+         rtp_sender_.IsRtpHeaderExtensionRegistered(
+             kRtpExtensionTransmissionTimeOffset);
+}
+
 // (TMMBR) Temporary Max Media Bit Rate.
 bool ModuleRtpRtcpImpl::TMMBR() const {
   return rtcp_sender_.TMMBR();