Add virtualKeyboardPrivate API permission to ChromeVox manifest.

chrome/browser/resources/chromeos/chromevox/manifest.json.jinja2

 {
   "key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDltVl1k15pjRzuZfMc3B69inxwm2bZeZ2O8/zFO+NluHnBm3GJ3fzdOoFGJd+M16I8p7zxxQyHeDMfWYASyCeB8XnUEDKjqNLQfCnncsANzHsYoEbYj2nEUML2P13b9q+AAvpCBpAJ4cZp81e9n1y/vbSXHE4385cgkKueItzikQIDAQAB",
   "manifest_version": 2,
   "name": "ChromeVox",
   "version": "1.0",
   "description": "ChromeVox - Giving Voice to Chrome.",
 {% if is_guest_manifest == '1' %}
   "incognito": "split",
 {% endif %}
   "background": {
 {% if use_chromevox_next == '1' %}
     "page": "cvox2/background/background.html"
 {% else %}
     "page": "chromevox/background/background.html"
 {% endif %}
   },
   "permissions": [
     "accessibilityPrivate",
     "automation",
     "bookmarks",
     "tabs",
     "experimental",
     "history",
     "tts",
     "systemPrivate",
     "brailleDisplayPrivate",
     "commandLinePrivate",
+    "virtualKeyboardPrivate",
     "<all_urls>"
   ],
   "content_scripts": [
     {
       "matches": [ "<all_urls>" ],
       "exclude_globs": [ "chrome-extension://mndnfokpggljbaajbnioimlmbfngpief/chromevox/background/background.html" ],
       "all_frames": true,
       "js": [
 {% if use_chromevox_next == '1' %}
         "closure/closure_preinit.js",
@@ skipping 12 matching lines @@
     "chromevox/injected/mathjax.js",
     "chromevox/injected/mathjax_external_util.js"
   ],
 {% if use_chromevox_next == '1' %}
   "automation": {
     "desktop": true
   },
 {% endif %}
   "default_locale": "en"
 }