Replace RegisterExternalDecoder in NetEq test VerifyTimestampPropagation.

Bug: webrtc:10080
Change-Id: Ie93f130863115c2d288cfd9f3e273a9fbc982ed6
Reviewed-on: https://webrtc-review.googlesource.com/c/112904
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25927}
diff --git a/test/BUILD.gn b/test/BUILD.gn
index 1557af2..19989e3 100644
--- a/test/BUILD.gn
+++ b/test/BUILD.gn
@@ -830,6 +830,7 @@
 rtc_source_set("audio_codec_mocks") {
   testonly = true
   sources = [
+    "audio_decoder_proxy_factory.h",
     "function_audio_decoder_factory.h",
     "mock_audio_decoder.cc",
     "mock_audio_decoder.h",