Reland "Rename stereo video codec to multiplex"

This is a reland of bbdabe50db0cf09f6007dda12a6476dc4602b174.

Original change's description:
> Rename stereo video codec to multiplex
>
> This CL only does the rename from"stereo" to multiplex". With this we have a
> better name that doesn't clash with audio's usage of stereo.
>
> Bug: webrtc:7671
> Change-Id: Iebc3fc20839025f1bc8bcf0e16141bf9744ef652
> Reviewed-on: https://webrtc-review.googlesource.com/43242
> Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
> Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21769}

TBR=niklas.enbom@webrtc.org

Bug: webrtc:7671
Change-Id: I6f38dc46126f279f334d52b56339b40acdc30511
Reviewed-on: https://webrtc-review.googlesource.com/45820
Reviewed-by: Emircan Uysaler <emircan@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21794}
diff --git a/test/video_codec_settings.h b/test/video_codec_settings.h
index 5dabc70..bdf8ca5 100644
--- a/test/video_codec_settings.h
+++ b/test/video_codec_settings.h
@@ -103,7 +103,7 @@
     case kVideoCodecRED:
     case kVideoCodecULPFEC:
     case kVideoCodecFlexfec:
-    case kVideoCodecStereo:
+    case kVideoCodecMultiplex:
     case kVideoCodecGeneric:
     case kVideoCodecUnknown:
       RTC_NOTREACHED();