feat: Support HTML-escaped cues in VTT

lib/text/vtt_text_parser.js

@@ -366,7 +366,8 @@ shaka.text.VttTextParser = class {
     end += timeOffset;
 
     // Get the payload.
-    const payload = text.slice(1).join('\n').trim();
+    const payload = VttTextParser.htmlUnescape_(
+        text.slice(1).join('\n').trim());
 
     let cue = null;
     if (styles.has('global')) {
@@ -843,6 +844,35 @@ shaka.text.VttTextParser = class {
 
     return (milliseconds / 1000) + seconds + (minutes * 60) + (hours * 3600);
   }
+
+  /**
+   * This method converts the HTML entities &, <, >, ", and
+   * ' in string to their corresponding characters.
+   *
+   * @param {!string} input
+   * @return {string}
+   * @private
+   */
+  static htmlUnescape_(input) {
+    /** Used to map HTML entities to characters. */
+    const htmlUnescapes = {
+      '&': '&',
+      '&lt;': '<',
+      '&gt;': '>',
+      '&quot;': '"',
+      '&#39;': '\'',
+    };
+
+    /** Used to match HTML entities and HTML characters. */
+    const reEscapedHtml = /&(?:amp|lt|gt|quot|#(0+)?39);/g;
+    const reHasEscapedHtml = RegExp(reEscapedHtml.source);
+    if (input && reHasEscapedHtml.test(input)) {
+      return input.replace(reEscapedHtml, (entity) => {
+        return htmlUnescapes[entity] || '\'';
+      });
+    }
+    return input || '';
+  }
 };
 
 /**