Guard chrome.audio usage with check

chrome.audio is not available on chromecast. This caused
the background script to stop working.  Adding a guard
to make sure the api is available before use.

Test: Manual
Bug: 903924
Change-Id: I9f162d2f80074e72e44deca6d2893255fff37d2e
Reviewed-on: https://chromium-review.googlesource.com/c/1330220
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609319}

chrome/browser/resources/chromeos/chromevox/cvox2/background/next_earcons.js

@@ -36,11 +36,15 @@ NextEarcons = function() {
   /** @private {boolean} */
   this.shouldPan_ = true;
 
-  chrome.audio.getDevices(
-      {isActive: true, streamTypes: [chrome.audio.StreamType.OUTPUT]},
-      this.updateShouldPanForDevices_.bind(this));
-  chrome.audio.onDeviceListChanged.addListener(
-      this.updateShouldPanForDevices_.bind(this));
+  if (chrome.audio) {
+    chrome.audio.getDevices(
+        {isActive: true, streamTypes: [chrome.audio.StreamType.OUTPUT]},
+        this.updateShouldPanForDevices_.bind(this));
+    chrome.audio.onDeviceListChanged.addListener(
+        this.updateShouldPanForDevices_.bind(this));
+  } else {
+    this.shouldPan_ = false;
+  }
 };
 
 NextEarcons.prototype = {