[Chromecast] Prepare for audio eraser on new device.

Bug: 70225530
Test: build and push OTA to device.
Change-Id: I3e77b8e0ea55084c499de36169d80a195c0de74a
Reviewed-on: https://chromium-review.googlesource.com/958225Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Linkun Chen <lkchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542559}

chromecast/media/cma/backend/video/cast_media_shlib.cc

@@ -9,6 +9,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chromecast/base/task_runner_impl.h"
+#include "chromecast/media/cma/backend/stream_mixer.h"
 #include "chromecast/public/graphics_types.h"
 #include "chromecast/public/media/media_capabilities_shlib.h"
 
@@ -77,5 +78,14 @@ bool MediaCapabilitiesShlib::IsSupportedAudioConfig(const AudioConfig& config) {
   return false;
 }
 
+void CastMediaShlib::AddLoopbackAudioObserver(LoopbackAudioObserver* observer) {
+  StreamMixer::Get()->AddLoopbackAudioObserver(observer);
+}
+
+void CastMediaShlib::RemoveLoopbackAudioObserver(
+    LoopbackAudioObserver* observer) {
+  StreamMixer::Get()->RemoveLoopbackAudioObserver(observer);
+}
+
 }  // namespace media
 }  // namespace chromecast