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}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_unittest.cc b/webrtc/modules/audio_coding/neteq/neteq_unittest.cc
index 4a6f505..24a8774 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_unittest.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_unittest.cc
@@ -349,7 +349,7 @@
                                              (output_sample_rate_ / 1000))));
     }
     // Get next packet.
-    packet_.reset(rtp_source_->NextPacket());
+    packet_ = rtp_source_->NextPacket();
   }
 
   // Get audio from NetEq.
@@ -387,7 +387,7 @@
       gen_ref ? webrtc::test::OutputPath() + "neteq_rtcp_stats.dat" : "";
   ResultSink rtcp_stats(rtcp_out_file);
 
-  packet_.reset(rtp_source_->NextPacket());
+  packet_ = rtp_source_->NextPacket();
   int i = 0;
   while (packet_) {
     std::ostringstream ss;