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/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 8490a60..775fd14 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -403,9 +403,11 @@
     "numerics/exp_filter.cc",
     "numerics/exp_filter.h",
     "numerics/percentile_filter.h",
+    "numerics/sequence_number_util.h",
   ]
   deps = [
     ":rtc_base_approved",
+    "../api:optional",
   ]
 }
 
@@ -996,6 +998,7 @@
     sources = [
       "numerics/exp_filter_unittest.cc",
       "numerics/percentile_filter_unittest.cc",
+      "numerics/sequence_number_util_unittest.cc",
     ]
     deps = [
       ":rtc_base_approved",