Bug 1766273 - Add PiP subtitle support for Disney+. r=mhowell

Subtitle structure of disney+ seems to be the following.

```
<div class="dss-hls-subtitle-overlay">
  <div class="dss-subtitle-renderer-wrapper">
    <div class="dss-subtitle-renderer-cue-positioning-box">
      <div class="dss-subtitle-renderer-cue-window">
        <span class="dss-subtitle-renderer-cue">
          <span class="dss-subtitle-renderer-line">...</span>
          <span class="dss-subtitle-renderer-line">...</span>
```

Differential Revision: https://phabricator.services.mozilla.com/D149849

browser/extensions/pictureinpicture/data/picture_in_picture_overrides.js

@@ -61,6 +61,12 @@ let AVAILABLE_PIP_OVERRIDES;
       },
     },
 
+    disneyplus: {
+      "https://*.disneyplus.com/*": {
+        videoWrapperScriptPath: "video-wrappers/disneyplus.js",
+      },
+    },
+
     funimation: {
       "https://*.funimation.com/*": {
         videoWrapperScriptPath: "video-wrappers/funimation.js",

browser/extensions/pictureinpicture/moz.build

@@ -31,6 +31,7 @@ FINAL_TARGET_FILES.features["pictureinpicture@mozilla.org"]["video-wrappers"] +=
     "video-wrappers/airmozilla.js",
     "video-wrappers/bbc.js",
     "video-wrappers/dailymotion.js",
+    "video-wrappers/disneyplus.js",
     "video-wrappers/funimation.js",
     "video-wrappers/hbomax.js",
     "video-wrappers/hotstar.js",

browser/extensions/pictureinpicture/video-wrappers/disneyplus.js

@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+class PictureInPictureVideoWrapper {
+  setCaptionContainerObserver(video, updateCaptionsFunction) {
+    let container = document.querySelector(".dss-hls-subtitle-overlay");
+
+    if (container) {
+      const callback = () => {
+        let textNodeList = container.querySelectorAll(
+          ".dss-subtitle-renderer-line"
+        );
+
+        if (!textNodeList.length) {
+          updateCaptionsFunction("");
+          return;
+        }
+
+        updateCaptionsFunction(
+          Array.from(textNodeList, x => x.textContent).join("\n")
+        );
+      };
+
+      // immediately invoke the callback function to add subtitles to the PiP window
+      callback();
+
+      let captionsObserver = new MutationObserver(callback);
+      captionsObserver.observe(container, {
+        attributes: false,
+        childList: true,
+        subtree: true,
+      });
+    }
+  }
+}
+
+this.PictureInPictureVideoWrapper = PictureInPictureVideoWrapper;