Split out counting unique rtp timestamps from packet_buffer

Bug: None
Change-Id: Ia6fd05f284e8304cf56ab9ddf944fb222a4c9573
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158676
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29656}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index dd202ce..39e66cf 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -130,6 +130,8 @@
     "timestamp_map.h",
     "timing.cc",
     "timing.h",
+    "unique_timestamp_counter.cc",
+    "unique_timestamp_counter.h",
     "video_codec_initializer.cc",
     "video_receiver2.cc",
     "video_receiver2.h",
@@ -849,6 +851,7 @@
       "test/stream_generator.cc",
       "test/stream_generator.h",
       "timing_unittest.cc",
+      "unique_timestamp_counter_unittest.cc",
       "utility/decoded_frames_history_unittest.cc",
       "utility/default_video_bitrate_allocator_unittest.cc",
       "utility/frame_dropper_unittest.cc",