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.h b/webrtc/modules/video_coding/packet_buffer.h
index caa81f6..75049b3 100644
--- a/webrtc/modules/video_coding/packet_buffer.h
+++ b/webrtc/modules/video_coding/packet_buffer.h
@@ -14,6 +14,7 @@
 #include <array>
 #include <vector>
 #include <map>
+#include <memory>
 #include <set>
 #include <queue>