Add aria-label for popup dialog in Assistant opt-in flow

Bug: 1024103
Test: Manual Test
Change-Id: I323573a32c3b3925e9840f1cabfe8029d5b5ac9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1928195Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718320}

chrome/browser/resources/chromeos/assistant_optin/assistant_third_party.js

@@ -78,16 +78,18 @@ Polymer({
     }
     e.preventDefault();
     this.lastFocusedElement = e.target;
-    this.showThirdPartyOverlay(e.target.href);
+    this.showThirdPartyOverlay(e.target.href, e.target.innerText);
   },
 
   /**
    * Shows third party information links in overlay dialog.
    * @param {string} url URL to show.
+   * @param {string} title Title of the dialog.
    */
-  showThirdPartyOverlay: function(url) {
+  showThirdPartyOverlay: function(url, title) {
     this.$['webview-container'].classList.add('overlay-loading');
     this.$['overlay-webview'].src = url;
+    this.$['third-party-overlay'].setTitleAriaLabel(title);
     this.$['third-party-overlay'].showModal();
     this.$['overlay-close-button'].focus();
   },

chrome/browser/resources/chromeos/assistant_optin/assistant_value_prop.js

@@ -164,6 +164,7 @@ Polymer({
         this.sanitizer_.sanitizeHtml(title);
     this.$['overlay-additional-info-text'].innerHTML =
         this.sanitizer_.sanitizeHtml(additionalInfo);
+    this.$['learn-more-overlay'].setTitleAriaLabel(title);
 
     this.$['learn-more-overlay'].showModal();
     this.$['overlay-close-button'].focus();

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js

@@ -193,6 +193,15 @@ Polymer({
     this.open = false;
   },
 
+  /**
+   * Set the title of the dialog for a11y reader.
+   * @param {string} title Title of the dialog.
+   */
+  setTitleAriaLabel: function(title) {
+    this.$.dialog.removeAttribute('aria-labelledby');
+    this.$.dialog.setAttribute('aria-label', title);
+  },
+
   /**
    * @private
    * @param {Event} e