Add SystemVolumeControl implementation for desktop.

We don't have support for alsa on borg, so this is needed to prevent an
endless stream of alsa related errors.

Bug: b/151360165
Change-Id: I765b98d637ed1741c16069024160c5748ec6cde9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150032
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Auto-Submit: Bryan Clark <bryanclark@google.com>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759369}

chromecast/media/cma/backend/desktop/BUILD.gn

@@ -25,10 +25,11 @@ cast_source_set("desktop") {
     "//media",
   ]
   if (enable_video_with_mixed_audio) {
+    sources += [ "desktop_system_volume_control.cc" ]
     deps += [
       "//chromecast/media/cma/backend:for_mixer_audio",
       "//chromecast/media/cma/backend:null_video",
-      "//chromecast/media/cma/backend/alsa:volume_control",
+      "//chromecast/media/cma/backend:public",
       "//chromecast/media/cma/backend/video:av_sync_video",
     ]
   } else {

chromecast/media/cma/backend/desktop/desktop_system_volume_control.cc

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <memory>
+
+#include "chromecast/media/cma/backend/system_volume_control.h"
+
+namespace chromecast {
+namespace media {
+
+class DesktopSystemVolumeControl : public SystemVolumeControl {
+ public:
+  explicit DesktopSystemVolumeControl(Delegate* delegate)
+      : delegate_(delegate) {}
+  ~DesktopSystemVolumeControl() override = default;
+
+  // SystemVolumeControl interface.
+  // Consider 'level' and 'volume' equivalent for simplicity.
+  float GetRoundtripVolume(float volume) override { return volume; }
+  float GetVolume() override { return volume_; }
+  void SetVolume(float level) override {
+    if (level != volume_) {
+      volume_ = level;
+      delegate_->OnSystemVolumeOrMuteChange(volume_, muted_);
+    }
+  }
+  bool IsMuted() override { return muted_; }
+  void SetMuted(bool muted) override {
+    if (muted != muted_) {
+      muted_ = muted;
+      delegate_->OnSystemVolumeOrMuteChange(volume_, muted_);
+    }
+  }
+  void SetPowerSave(bool power_save_on) override {}
+  void SetLimit(float limit) override {}
+
+ private:
+  Delegate* delegate_;
+  float volume_ = 0.0f;
+  bool muted_ = false;
+};
+
+// static
+std::unique_ptr<SystemVolumeControl> SystemVolumeControl::Create(
+    Delegate* delegate) {
+  return std::make_unique<DesktopSystemVolumeControl>(delegate);
+}
+
+}  // namespace media
+}  // namespace chromecast