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}
diff --git a/modules/video_coding/video_codec_initializer_unittest.cc b/modules/video_coding/video_codec_initializer_unittest.cc
index ed6ad9c..e8925e1 100644
--- a/modules/video_coding/video_codec_initializer_unittest.cc
+++ b/modules/video_coding/video_codec_initializer_unittest.cc
@@ -79,7 +79,7 @@
           webrtc::VideoEncoderConfig::Vp8EncoderSpecificSettings>(vp8_settings);
       settings_.payload_name = kVp8PayloadName;
       settings_.payload_type = kVp8PayloadType;
-    } else if (type == VideoCodecType::kVideoCodecMultiplex) {
+    } else if (type == VideoCodecType::kVideoCodecStereo) {
     } else {
       ADD_FAILURE() << "Unexpected codec type: " << type;
     }
@@ -94,7 +94,7 @@
                                            &bitrate_allocator_out_)) {
       return false;
     }
-    if (codec_out_.codecType == VideoCodecType::kVideoCodecMultiplex)
+    if (codec_out_.codecType == VideoCodecType::kVideoCodecStereo)
       return true;
     // Make sure temporal layers instances have been created.
     if (codec_out_.codecType == VideoCodecType::kVideoCodecVP8) {
@@ -257,8 +257,8 @@
             bitrate_allocation.GetBitrate(1, 1));
 }
 
-TEST_F(VideoCodecInitializerTest, SingleStreamMultiplexCodec) {
-  SetUpFor(VideoCodecType::kVideoCodecMultiplex, 1, 1, true);
+TEST_F(VideoCodecInitializerTest, SingleStreamStereoCodec) {
+  SetUpFor(VideoCodecType::kVideoCodecStereo, 1, 1, true);
   streams_.push_back(DefaultStream());
   EXPECT_TRUE(InitializeCodec());
 }