Wrap new API in a try-catch.

Since the webstore release cannot (at the moment) be tied to any particular version of Chrome, something like this is necessary to prevent us from crashing.

TEST=load into Chrome stable; verify we still come up talking.

Review URL: https://codereview.chromium.org/1514483006

Cr-Commit-Position: refs/heads/master@{#364471}

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

@@ -42,8 +42,12 @@ LiveRegions = function(chromeVoxState) {
    */
   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) {
+  }
 };
 
 /**