Opus implementation of the AudioDecoderFactoryTemplate API

(This got reverted because of a problem with the Opus encoder parts.
Re-landing without changes.)

BUG=webrtc:7837

Review-Url: https://codereview.webrtc.org/2950453002
Cr-Commit-Position: refs/heads/master@{#18855}
diff --git a/webrtc/api/audio_codecs/opus/BUILD.gn b/webrtc/api/audio_codecs/opus/BUILD.gn
index b82f496..c7f7ac8 100644
--- a/webrtc/api/audio_codecs/opus/BUILD.gn
+++ b/webrtc/api/audio_codecs/opus/BUILD.gn
@@ -40,3 +40,16 @@
     "../../../modules/audio_coding:webrtc_opus",
   ]
 }
+
+rtc_static_library("audio_decoder_opus") {
+  sources = [
+    "audio_decoder_opus.cc",
+    "audio_decoder_opus.h",
+  ]
+  deps = [
+    "..:audio_codecs_api",
+    "../../..:webrtc_common",
+    "../../../base:rtc_base_approved",
+    "../../../modules/audio_coding:webrtc_opus",
+  ]
+}