Move sequence_number_utils.h to rtc_base/

Bug: webrtc:8440
Change-Id: I36e70da6ce70b95db7d3fce8b0013bff5c795bfc
Reviewed-on: https://webrtc-review.googlesource.com/14860
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20429}
diff --git a/modules/video_coding/packet_buffer.h b/modules/video_coding/packet_buffer.h
index 6604c36..ca499bf 100644
--- a/modules/video_coding/packet_buffer.h
+++ b/modules/video_coding/packet_buffer.h
@@ -18,8 +18,8 @@
 #include "modules/include/module_common_types.h"
 #include "modules/video_coding/packet.h"
 #include "modules/video_coding/rtp_frame_reference_finder.h"
-#include "modules/video_coding/sequence_number_util.h"
 #include "rtc_base/criticalsection.h"
+#include "rtc_base/numerics/sequence_number_util.h"
 #include "rtc_base/scoped_ref_ptr.h"
 #include "rtc_base/thread_annotations.h"