ChromeVox Hint: Ensure the hint is only given once.

This change fixes an issue where giveChromeVoxHint_ was being called
several times. This is likely because it's bound to an event listener,
which isn't removed until several calls have been queued up. To
prevent this, use a variable to track when the hint is given.

Fixed: 1162246
Change-Id: I9e9ed3ac5ce83e7011d29b2edc392d019289803a
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613366Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841541}

chrome/browser/resources/chromeos/login/oobe_welcome.js

@@ -129,7 +129,13 @@ Polymer({
      * @private {number}
      * @const
      */
-    DEFAULT_CHROMEVOX_HINT_TIMEOUT_MS_: {type: Number, value: 40 * 1000}
+    DEFAULT_CHROMEVOX_HINT_TIMEOUT_MS_: {type: Number, value: 40 * 1000},
+
+    /**
+     * Tracks if we've given the ChromeVox hint yet.
+     * @private
+     */
+    chromeVoxHintGiven_: {type: Boolean, value: false}
   },
 
   /** Overridden from LoginScreenBehavior. */
@@ -668,7 +674,7 @@ Polymer({
    * @private
    */
   onVoiceNotLoaded_() {
-    if (!this.voicesChangedListenerMaybeGiveChromeVoxHint_) {
+    if (this.voicesChangedListenerMaybeGiveChromeVoxHint_ === undefined) {
       // Add voiceschanged listener that tries to give the hint when new voices
       // are loaded.
       this.voicesChangedListenerMaybeGiveChromeVoxHint_ =
@@ -700,6 +706,14 @@ Polymer({
    * @private
    */
   giveChromeVoxHint_(locale, options, isDefaultHint) {
+    if (this.chromeVoxHintGiven_) {
+      // Only give the hint once.
+      // Due to event listeners/timeouts, there is the chance that this gets
+      // called multiple times.
+      return;
+    }
+
+    this.chromeVoxHintGiven_ = true;
     if (isDefaultHint) {
       console.warn(
           'No voice available for ' + loadTimeData.getString('language') +