feat(UI): Remove copyStyleSheets (#5273)

Document Picture-in-Picture API owners are [removing `copyStyleSheets` option](WICG/document-picture-in-picture#79) as it can be done in JavaScript.
This PR removes this option and adds code to copy the style sheets of the PiP window.

externs/pictureinpicture.js

@@ -55,7 +55,6 @@ HTMLMediaElement.prototype.webkitPresentationMode;
  * @typedef {{
  *   width: (number|undefined),
  *   height: (number|undefined),
- *   copyStyleSheets: (boolean|undefined),
  * }}
  */
 var DocumentPictureInPictureOptions;

ui/pip_button.js

@@ -164,9 +164,11 @@ shaka.ui.PipButton = class extends shaka.ui.Element {
     const pipWindow = await window.documentPictureInPicture.requestWindow({
       width: rectPipPlayer.width,
       height: rectPipPlayer.height,
-      copyStyleSheets: true,
     });
 
+    // Copy style sheets to the Picture-in-Picture window.
+    this.copyStyleSheetsToWindow_(pipWindow);
+
     // Add placeholder for the player.
     const parentPlayer = pipPlayer.parentNode || document.body;
     const placeholder = this.videoContainer_.cloneNode(true);
@@ -190,6 +192,33 @@ shaka.ui.PipButton = class extends shaka.ui.Element {
     });
   }
 
+  /** @private */
+  copyStyleSheetsToWindow_(win) {
+    const styleSheets = /** @type {!Iterable<*>} */(document.styleSheets);
+    const allCSS = [...styleSheets]
+        .map((sheet) => {
+          try {
+            return [...sheet.cssRules].map((rule) => rule.cssText).join('');
+          } catch (e) {
+            const link = /** @type {!HTMLLinkElement} */(
+              document.createElement('link'));
+
+            link.rel = 'stylesheet';
+            link.type = sheet.type;
+            link.media = sheet.media;
+            link.href = sheet.href;
+            win.document.head.appendChild(link);
+          }
+          return '';
+        })
+        .filter(Boolean)
+        .join('\n');
+    const style = document.createElement('style');
+
+    style.textContent = allCSS;
+    win.document.head.appendChild(style);
+  }
+
   /** @private */
   onEnterPictureInPicture_() {
     const LocIds = shaka.ui.Locales.Ids;