Let PacketSource::NextPacket() return an std::unique_ptr

The return type of PacketSource::NextPacket() is changed from a naked
pointer to an std::uniqe_ptr. The interface contract was and still is
that the ownership is passed from the callee to the caller, but a
unique_ptr makes this explicit.

BUG=webrtc:2692

Review-Url: https://codereview.webrtc.org/2005873002
Cr-Commit-Position: refs/heads/master@{#12884}

webrtc/modules/audio_coding/neteq/tools/rtp_analyze.cc

@@ -107,7 +107,7 @@ int main(int argc, char* argv[]) {
   int cycles = -1;
   std::unique_ptr<webrtc::test::Packet> packet;
   while (true) {
-    packet.reset(file_source->NextPacket());
+    packet = file_source->NextPacket();
     if (!packet.get()) {
       // End of file reached.
       break;