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/BUILD.gn b/modules/video_coding/BUILD.gn
index 416cfb4..c467ea9 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -94,7 +94,7 @@
     ":video_coding_utility",
     ":webrtc_h264",
     ":webrtc_i420",
-    ":webrtc_stereo",
+    ":webrtc_multiplex",
     ":webrtc_vp8",
     ":webrtc_vp8_helpers",
     ":webrtc_vp9",
@@ -289,14 +289,14 @@
   ]
 }
 
-rtc_static_library("webrtc_stereo") {
+rtc_static_library("webrtc_multiplex") {
   sources = [
-    "codecs/stereo/include/multiplex_encoded_image_packer.h",
-    "codecs/stereo/include/stereo_decoder_adapter.h",
-    "codecs/stereo/include/stereo_encoder_adapter.h",
-    "codecs/stereo/multiplex_encoded_image_packer.cc",
-    "codecs/stereo/stereo_decoder_adapter.cc",
-    "codecs/stereo/stereo_encoder_adapter.cc",
+    "codecs/multiplex/include/multiplex_decoder_adapter.h",
+    "codecs/multiplex/include/multiplex_encoded_image_packer.h",
+    "codecs/multiplex/include/multiplex_encoder_adapter.h",
+    "codecs/multiplex/multiplex_decoder_adapter.cc",
+    "codecs/multiplex/multiplex_encoded_image_packer.cc",
+    "codecs/multiplex/multiplex_encoder_adapter.cc",
   ]
 
   if (!build_with_chromium && is_clang) {
@@ -545,7 +545,7 @@
 
     sources = [
       "codecs/h264/test/h264_impl_unittest.cc",
-      "codecs/stereo/test/stereo_adapter_unittest.cc",
+      "codecs/multiplex/test/multiplex_adapter_unittest.cc",
       "codecs/test/videoprocessor_integrationtest.cc",
       "codecs/test/videoprocessor_integrationtest.h",
       "codecs/test/videoprocessor_integrationtest_libvpx.cc",
@@ -561,7 +561,7 @@
       ":video_coding",
       ":video_coding_utility",
       ":webrtc_h264",
-      ":webrtc_stereo",
+      ":webrtc_multiplex",
       ":webrtc_vp8",
       ":webrtc_vp8_helpers",
       ":webrtc_vp9",