Distinguish between missing packet and send failure.

This CL introduces three-value enum, in order to be able to distinguish
between send success, send failure, and invalid states such as missing
packet or invalid ssrc.

The behavior is unchanged in this CL, a follow-up will change the pacer
to not consume media budget on invalid states.

Bug: webrtc:8052,webrtc:8975
Change-Id: I1c9e2226f995356daa538d3d3cf44945f35e0133
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135165
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27923}

modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -547,5 +547,12 @@ class SendPacketObserver {
                             uint32_t ssrc) = 0;
 };
 
+// Status returned from TimeToSendPacket() family of callbacks.
+enum class RtpPacketSendResult {
+  kSuccess,               // Packet sent OK.
+  kTransportUnavailable,  // Network unavailable, try again later.
+  kPacketNotFound  // SSRC/sequence number does not map to an available packet.
+};
+
 }  // namespace webrtc
 #endif  // MODULES_RTP_RTCP_INCLUDE_RTP_RTCP_DEFINES_H_