fix: add missing properties to PlayButton type

AUTHORS

@@ -88,6 +88,7 @@ Swank Motion Pictures Inc. <*@swankmp.com>
 TalkTalk Plc <*@talktalkplc.com>
 Tatsiana Gelahova <tatsiana.gelahova@gmail.com>
 Tian Shao <tyrelltle@gmail.com>
+Thomas Champion <thchampi@gmail.com>
 Tomas Tichy <mr.tichyt@gmail.com>
 Tomohiro Matsuzawa <thmatuza75@hotmail.com>
 Toshihiro Suzuki <t.suzuki326@gmail.com>

CONTRIBUTORS

@@ -128,6 +128,7 @@ Thomas Stephens <thomas@ustudio.com>
 Tian Shao <tyrelltle@gmail.com>
 Tim Plummer <objelisks@google.com>
 Timothy Drews <tdrews@google.com>
+Thomas Champion <thchampi@gmail.com>
 Tomas Bucher <collapsingtesseract@gmail.com>
 Tomas Tichy <mr.tichyt@gmail.com>
 Tomohiro Matsuzawa <thmatuza75@hotmail.com>

ui/externs/ui.js

@@ -497,3 +497,27 @@ shaka.extern.IUISeekBar.Factory = class {
    */
   create(rootElement, controls) {}
 };
+
+/**
+ * @interface
+ * @exportDoc
+ */
+shaka.extern.IUIPlayButton = class {
+  /**
+   * @param {!HTMLElement} parent
+   * @param {!shaka.ui.Controls} controls
+   */
+  constructor(parent, controls) {
+    /**
+     * @protected {!HTMLButtonElement}
+     * @exportDoc
+     */
+    this.button;
+  }
+
+  /** @return {boolean} */
+  isPaused() {}
+
+  /** @return {boolean} */
+  isEnded() {}
+};

ui/play_button.js

@@ -16,6 +16,7 @@ goog.requireType('shaka.ui.Controls');
 
 /**
  * @extends {shaka.ui.Element}
+ * @implements {shaka.extern.IUIPlayButton}
  * @export
  */
 shaka.ui.PlayButton = class extends shaka.ui.Element {
@@ -95,6 +96,7 @@ shaka.ui.PlayButton = class extends shaka.ui.Element {
   /**
    * @return {boolean}
    * @protected
+   * @override
    */
   isPaused() {
     if (this.ad && this.ad.isLinear()) {
@@ -107,6 +109,7 @@ shaka.ui.PlayButton = class extends shaka.ui.Element {
   /**
    * @return {boolean}
    * @protected
+   * @override
    */
   isEnded() {
     if (this.ad && this.ad.isLinear()) {