Add NetEqInput::PacketData::ToString method

This new method prints information about the packet.

BUG=webrtc:7467

Review-Url: https://codereview.webrtc.org/2844283002
Cr-Commit-Position: refs/heads/master@{#17922}

webrtc/modules/audio_coding/neteq/tools/neteq_input.h

@@ -13,6 +13,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <string>
 
 #include "webrtc/base/buffer.h"
 #include "webrtc/base/optional.h"
@@ -27,6 +28,8 @@ namespace test {
 class NetEqInput {
  public:
   struct PacketData {
+    std::string ToString() const;
+
     RTPHeader header;
     rtc::Buffer payload;
     double time_ms;