Replace the remaining scoped_ptr with unique_ptr in webrtc/modules/

(This is a re-land of https://codereview.webrtc.org/1921233002, which
got reverted for breaking Chromium.)

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1923133002

Cr-Commit-Position: refs/heads/master@{#12522}
diff --git a/webrtc/modules/video_coding/packet_buffer_unittest.cc b/webrtc/modules/video_coding/packet_buffer_unittest.cc
index 28c62c2..eb0a03b 100644
--- a/webrtc/modules/video_coding/packet_buffer_unittest.cc
+++ b/webrtc/modules/video_coding/packet_buffer_unittest.cc
@@ -10,6 +10,7 @@
 
 #include <cstring>
 #include <limits>
+#include <memory>
 
 #include "webrtc/modules/video_coding/frame_object.h"
 #include "webrtc/modules/video_coding/packet_buffer.h"