Make MediaSession a runtime-enabled feature on Desktop.

content/browser/media/session/media_session_delegate_default.cc

+// Copyright 2016 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 "content/browser/media/session/media_session_delegate.h"
+
+#include "base/command_line.h"
+#include "content/browser/web_contents/web_contents_impl.h"
+#include "media/base/media_switches.h"
+
+namespace content {
+
+namespace {
+
+// MediaSessionDelegateDefault is the default implementation of
+// MediaSessionDelegate which only handles audio focus between WebContents.
+class MediaSessionDelegateDefault : public MediaSessionDelegate {
+ public:
+  explicit MediaSessionDelegateDefault(MediaSession* media_session);
+
+  // MediaSessionDelegate implementation.
+  bool RequestAudioFocus(MediaSession::Type type) override;
+  void AbandonAudioFocus() override;
+
+ private:
+  // Weak pointer because |this| is owned by |media_session_|.
+  MediaSession* media_session_;
+};
+
+}  // anonymous namespace
+
+MediaSessionDelegateDefault::MediaSessionDelegateDefault(
+    MediaSession* media_session)
+    : media_session_(media_session) {
+}
+
+bool MediaSessionDelegateDefault::RequestAudioFocus(MediaSession::Type) {
+  if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kEnableDefaultMediaSession)) {
+    return true;
+  }
+
+  for (WebContentsImpl* web_contents : WebContentsImpl::GetAllWebContents()) {
+    MediaSession* media_session = MediaSession::Get(web_contents);
+    if (media_session == media_session_ || !media_session->IsActive())
+      continue;
+    media_session->Suspend(MediaSession::SuspendType::SYSTEM);
+  }
+  return true;
+}
+
+void MediaSessionDelegateDefault::AbandonAudioFocus() {
+}
+
+// static
+scoped_ptr<MediaSessionDelegate> MediaSessionDelegate::Create(
+    MediaSession* media_session) {
+  return scoped_ptr<MediaSessionDelegate>(
+      new MediaSessionDelegateDefault(media_session));
+}
+
+}  // namespace content