PacketBuffer now correctly cast sequence numbers to uint16_t.

BUG=webrtc:5514

Review-Url: https://codereview.webrtc.org/2603223002
Cr-Commit-Position: refs/heads/master@{#15885}
diff --git a/webrtc/modules/video_coding/packet_buffer.cc b/webrtc/modules/video_coding/packet_buffer.cc
index 03aeb5a..42fa8a8 100644
--- a/webrtc/modules/video_coding/packet_buffer.cc
+++ b/webrtc/modules/video_coding/packet_buffer.cc
@@ -187,7 +187,7 @@
   if (!sequence_buffer_[prev_index].used)
     return false;
   if (sequence_buffer_[prev_index].seq_num !=
-      sequence_buffer_[index].seq_num - 1) {
+      static_cast<uint16_t>(sequence_buffer_[index].seq_num - 1)) {
     return false;
   }
   if (sequence_buffer_[prev_index].continuous)