Adding integration test for unsignaled inbound RTP stream stats.

webrtc/pc/test/mockpeerconnectionobservers.h

Index: webrtc/pc/test/mockpeerconnectionobservers.h
diff --git a/webrtc/pc/test/mockpeerconnectionobservers.h b/webrtc/pc/test/mockpeerconnectionobservers.h
index 94996d004f1d8e8fc5c0d5fc26371e1b3a70a8f5..fb4c00419aa3ac043261dd07bc37dfeb8f32865f 100644
--- a/webrtc/pc/test/mockpeerconnectionobservers.h
+++ b/webrtc/pc/test/mockpeerconnectionobservers.h
@@ -225,6 +225,25 @@ class MockStatsObserver : public webrtc::StatsObserver {
   } stats_;
 };
 
+// Helper class that just stores the report from the callback.
+class MockRTCStatsCollectorCallback : public webrtc::RTCStatsCollectorCallback {
+ public:
+  rtc::scoped_refptr<const RTCStatsReport> report() { return report_; }
+
+  bool called() const { return called_; }
+
+ protected:
+  void OnStatsDelivered(
+      const rtc::scoped_refptr<const RTCStatsReport>& report) override {
+    report_ = report;
+    called_ = true;
+  }
+
+ private:
+  bool called_ = false;
+  rtc::scoped_refptr<const RTCStatsReport> report_;
+};
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_PC_TEST_MOCKPEERCONNECTIONOBSERVERS_H_