chrome.audio API: treat mute as system wide property

extensions/test/data/api_test/audio/input_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.assertTrue(isInput);
           chrome.test.assertTrue(isMuted);
         });
+    chrome.test.listenOnce(
+        chrome.audio.onMuteStateChanged,
+        function(evt) {
+          chrome.test.assertEq('INPUT', evt.streamType);
+          chrome.test.assertTrue(evt.value);
+        });
   }
 ]);
 
 chrome.test.sendMessage('loaded');