Wrap new API in a try-catch.

chrome/browser/resources/chromeos/chromevox/cvox2/background/live_regions.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.
 
 /**
  * @fileoverview Implements support for live regions in ChromeVox Next.
  */
 
 goog.provide('LiveRegions');
 
@@ skipping 24 matching lines @@
   this.lastLiveRegionTime_ = new Date(0);
 
   /**
    * Set of nodes that have been announced as part of a live region since
    *     |this.lastLiveRegionTime_|, to prevent duplicate announcements.
    * @type {!WeakSet<AutomationNode>}
    * @private
    */
   this.liveRegionNodeSet_ = new WeakSet();
 
-  chrome.automation.addTreeChangeObserver(
-      'liveRegionTreeChanges', this.onTreeChange.bind(this));
+  // API only exists >= m49. Prevent us from crashing.
+  try {
+    chrome.automation.addTreeChangeObserver(
+        'liveRegionTreeChanges', this.onTreeChange.bind(this));
+  } catch (e) {
+  }
 };
 
 /**
  * Live region events received in fewer than this many milliseconds will
  * queue, otherwise they'll be output with a category flush.
  * @type {number}
  * @const
  */
 LiveRegions.LIVE_REGION_QUEUE_TIME_MS = 500;
 
@@ skipping 93 matching lines @@
         return;
       parent = parent.parent;
     }
 
     this.liveRegionNodeSet_.add(node);
     output.go();
   },
 };
 
 });  // goog.scope