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/modules/video_coding/video_codec_initializer_unittest.cc b/modules/video_coding/video_codec_initializer_unittest.cc
index e8925e1..ed6ad9c 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::kVideoCodecStereo) {
+    } else if (type == VideoCodecType::kVideoCodecMultiplex) {
     } else {
       ADD_FAILURE() << "Unexpected codec type: " << type;
     }
@@ -94,7 +94,7 @@
                                            &bitrate_allocator_out_)) {
       return false;
     }
-    if (codec_out_.codecType == VideoCodecType::kVideoCodecStereo)
+    if (codec_out_.codecType == VideoCodecType::kVideoCodecMultiplex)
       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, SingleStreamStereoCodec) {
-  SetUpFor(VideoCodecType::kVideoCodecStereo, 1, 1, true);
+TEST_F(VideoCodecInitializerTest, SingleStreamMultiplexCodec) {
+  SetUpFor(VideoCodecType::kVideoCodecMultiplex, 1, 1, true);
   streams_.push_back(DefaultStream());
   EXPECT_TRUE(InitializeCodec());
 }