Add send transports to individual webrtc::Call streams.

webrtc/video/transport_adapter.cc

Index: webrtc/video/transport_adapter.cc
diff --git a/webrtc/video/transport_adapter.cc b/webrtc/video/transport_adapter.cc
index 7b5a6962c4483d19b3af13e1028e4f716480d2f3..225d4367259c22fe5455ad2e752c8960c87c2410 100644
--- a/webrtc/video/transport_adapter.cc
+++ b/webrtc/video/transport_adapter.cc
@@ -10,11 +10,15 @@
 
 #include "webrtc/video/transport_adapter.h"
 
+#include "webrtc/base/checks.h"
+
 namespace webrtc {
 namespace internal {
 
 TransportAdapter::TransportAdapter(newapi::Transport* transport)
-    : transport_(transport), enabled_(0) {}
+    : transport_(transport), enabled_(0) {
+  DCHECK(nullptr != transport);
+}
 
 int TransportAdapter::SendPacket(int /*channel*/,
                                  const void* packet,