ChromeVox: Add validation check in phonetic data api.

This change adds a validation check in ChromeVox's phonetic data api to
avoid potential crashes.

Change-Id: I47ef89e8a514c9227208e8821536ee53017c3c3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850357Reviewed-by: Randy Rossi <rmrossi@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704848}

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

@@ -58,5 +58,8 @@ PhoneticData.getPhoneticDisambiguation = function(language, character) {
   // language code to do a lookup.
   if (!PhoneticData.phoneticMap_[language])
     language = language.split('-')[0];
+  // If language still isn't in the map, return empty string.
+  if (!PhoneticData.phoneticMap_[language])
+    return '';
   return PhoneticData.phoneticMap_[language][character] || '';
 };