Pass VideoDecoderParams to VideoDecoderFactory and add SSRC to RtpEncodingParameters.

VideoDecoderParams contains the id of the receive video
stream. Motivation behind this change is to enable down
stream apps easier map raw non-decoded data to incoming
streams.

BUG=b/28636393

Review-Url: https://codereview.webrtc.org/2052233002
Cr-Commit-Position: refs/heads/master@{#13250}
diff --git a/webrtc/api/rtpparameters.h b/webrtc/api/rtpparameters.h
index 5c79ab4..13704dc 100644
--- a/webrtc/api/rtpparameters.h
+++ b/webrtc/api/rtpparameters.h
@@ -14,16 +14,20 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/base/optional.h"
+
 namespace webrtc {
 
 // These structures are defined as part of the RtpSender interface.
 // See http://w3c.github.io/webrtc-pc/#rtcrtpsender-interface for details.
 struct RtpEncodingParameters {
+  rtc::Optional<uint32_t> ssrc;
   bool active = true;
   int max_bitrate_bps = -1;
 
   bool operator==(const RtpEncodingParameters& o) const {
-    return active == o.active && max_bitrate_bps == o.max_bitrate_bps;
+    return ssrc == o.ssrc && active == o.active &&
+           max_bitrate_bps == o.max_bitrate_bps;
   }
   bool operator!=(const RtpEncodingParameters& o) const {
     return !(*this == o);