Re-enable audio tests

Disabled on some platforms in this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1100173

Bug: 110226305
Test: Run cast_audio_unittests on desktop, internal CQ

Change-Id: Ib1f3c275a53a47539d66c085d6b7c1a3d4708268
Reviewed-on: https://chromium-review.googlesource.com/1102981Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Mina Almasry <almasrymina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570863}

chromecast/BUILD.gn

@@ -96,7 +96,7 @@ cast_test_group("cast_tests") {
     ]
   }
 
-  if ((is_linux || is_fuchsia) && (use_alsa || enable_video_with_mixed_audio)) {
+  if (is_linux || is_fuchsia) {
     tests += [
       "//chromecast/media/cma/backend:cast_audio_backend_unittests",
       "//chromecast/media/cma/backend:cast_av_sync_backend_unittests",

chromecast/media/cma/backend/mixer_output_stream_dummy.cc

@@ -5,6 +5,7 @@
 #include <memory>
 
 #include "base/macros.h"
+#include "chromecast/media/cma/backend/system_volume_control.h"
 #include "chromecast/public/media/mixer_output_stream.h"
 
 namespace chromecast {
@@ -44,5 +45,13 @@ std::unique_ptr<MixerOutputStream> MixerOutputStream::Create() {
   return std::make_unique<MixerOutputStreamDummy>();
 }
 
+// static
+std::unique_ptr<SystemVolumeControl> SystemVolumeControl::Create(
+    Delegate* delegate) {
+  // No tests currently actually call this, so we don't need a real dummy
+  // implementation here.
+  return nullptr;
+}
+
 }  // namespace media
 }  // namespace chromecast