Revert "Rename stereo video codec to multiplex"

This reverts commit bbdabe50db0cf09f6007dda12a6476dc4602b174.

Reason for revert: This breaks the internal build.

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=sprang@webrtc.org,niklas.enbom@webrtc.org,qiangchen@chromium.org,emircan@webrtc.org

Change-Id: Icf019cb09e07de45821d31d7d8ea7707d01346ee
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:7671
Reviewed-on: https://webrtc-review.googlesource.com/44360
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21774}

common_types.h

@@ -485,7 +485,7 @@ enum VideoCodecType {
   kVideoCodecULPFEC,
   kVideoCodecFlexfec,
   kVideoCodecGeneric,
-  kVideoCodecMultiplex,
+  kVideoCodecStereo,
   kVideoCodecUnknown
 };