Add multiplex case to codec_tests

Bug: webrtc:7671
Change-Id: I38de24ea300d69757f5bd07111abc83b1f042751
Reviewed-on: https://webrtc-review.googlesource.com/50700
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21991}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 144987f..0a7b331 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -341,6 +341,7 @@
       "../modules/video_coding:video_codec_interface",
       "../modules/video_coding:video_coding_utility",
       "../modules/video_coding:webrtc_h264",
+      "../modules/video_coding:webrtc_multiplex",
       "../modules/video_coding:webrtc_vp8_helpers",
       "../modules/video_coding:webrtc_vp9",
       "../rtc_base:checks",