chrome.audio API: treat mute as system wide property

extensions/test/data/api_test/audio/output_mute_change/background.js

 // Copyright 2015 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.
 
-chrome.test.runTests([
-  function waitForMuteChangedEventTests() {
-    chrome.test.listenOnce(
-        chrome.audio.OnMuteChanged,
-        function(isInput, isMuted) {
-          chrome.test.assertFalse(isInput);
-          chrome.test.assertFalse(isMuted);
-        });
-  }
-]);
+chrome.test.runTests([function waitForMuteChangedEventTests() {
+  chrome.test.listenOnce(chrome.audio.onMuteChanged, function(evt) {
+    chrome.test.assertEq('OUTPUT', evt.streamType);
+    chrome.test.assertFalse(evt.isMuted);
+  });
+}]);
 
 chrome.test.sendMessage('loaded');