Generate track stats when SSRC=0

This will generate an all-zeroes track stat when the sender
has not yet been connected (SSRC has not been assigned).

Bug: webrtc:8673
Change-Id: Id59e6941bc87eba6bb33b4d2a8fd808d985052c7
Reviewed-on: https://webrtc-review.googlesource.com/43080
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21734}
diff --git a/media/base/mediachannel.h b/media/base/mediachannel.h
index 32ab479..3b78144 100644
--- a/media/base/mediachannel.h
+++ b/media/base/mediachannel.h
@@ -311,11 +311,15 @@
     }
     return retval;
   }
+  // Returns true if the media has been connected.
+  bool connected() const { return local_stats.size() > 0; }
   // Utility accessor for clients that make the assumption only one ssrc
   // exists per media.
   // This will eventually go away.
+  // Call sites that compare this to zero should use connected() instead.
+  // https://bugs.webrtc.org/8694
   uint32_t ssrc() const {
-    if (local_stats.size() > 0) {
+    if (connected()) {
       return local_stats[0].ssrc;
     } else {
       return 0;
@@ -351,11 +355,15 @@
     }
     return retval;
   }
+  // Returns true if the media has been connected.
+  bool connected() const { return local_stats.size() > 0; }
   // Utility accessor for clients that make the assumption only one ssrc
   // exists per media.
   // This will eventually go away.
+  // Call sites that compare this to zero should use connected();
+  // https://bugs.webrtc.org/8694
   uint32_t ssrc() const {
-    if (local_stats.size() > 0) {
+    if (connected()) {
       return local_stats[0].ssrc;
     } else {
       return 0;