Revert "SeqNumUnwrapper::Unwrap now returns int64_t instead of uint64_t."

This reverts commit b0f968a761b715da4cf81e4b9c3cab0ccd322cf2.

Reason for revert: Need to update DecodedFramesHistory to manage negative picture IDs.

Original change's description:
> SeqNumUnwrapper::Unwrap now returns int64_t instead of uint64_t.
> 
> Bug: webrtc:10263
> Change-Id: Idaeae6be01bd4eba0691226c958d70e114161ffd
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127295
> Commit-Queue: Philip Eliasson <philipel@webrtc.org>
> Reviewed-by: Johannes Kron <kron@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#27129}

TBR=kwiberg@webrtc.org,eladalon@webrtc.org,terelius@webrtc.org,philipel@webrtc.org,kron@webrtc.org

Change-Id: I529bb0475bd21a80fa244278aff1fd912a85c169
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:10263
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127885
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27135}
diff --git a/modules/video_coding/video_packet_buffer_unittest.cc b/modules/video_coding/video_packet_buffer_unittest.cc
index 9d8a3b0..3ef5624 100644
--- a/modules/video_coding/video_packet_buffer_unittest.cc
+++ b/modules/video_coding/video_packet_buffer_unittest.cc
@@ -501,7 +501,7 @@
   uint16_t seq_num = kFirstSeqNum;
 
   // Loop until seq_num wraps around.
-  SeqNumUnwrapper<uint16_t> unwrapper;
+  SeqNumUnwrapper<uint16_t> unwrapper(0);
   while (unwrapper.Unwrap(seq_num) < std::numeric_limits<uint16_t>::max()) {
     Insert(seq_num++, kKeyFrame, kFirst, kNotLast, 0, nullptr, timestamp);
     for (int i = 0; i < 5; ++i) {