Auto-update the mediacapture-fromelement IDL file (#11575)

* Add mediacapture-fromelement IDL file
* Change test to use that IDL file

interfaces/mediacapture-fromelement.idl

@@ -0,0 +1,16 @@
+// GENERATED CONTENT - DO NOT EDIT
+// Content of this file was automatically extracted from the
+// "Media Capture from DOM Elements" spec.
+// See: https://w3c.github.io/mediacapture-fromelement/
+
+partial interface HTMLMediaElement {
+    MediaStream captureStream();
+};
+partial interface HTMLCanvasElement {
+    MediaStream captureStream(optional double frameRequestRate);
+};
+[Exposed=Window]
+interface CanvasCaptureMediaStreamTrack : MediaStreamTrack {
+    readonly attribute HTMLCanvasElement canvas;
+    void requestFrame();
+};

mediacapture-fromelement/idlharness.html

@@ -13,33 +13,16 @@
   <media id='media' width=10 height=10/>
   <canvas id='canvas' width=10 height=10/>
 
-  <pre id="idl" style="display: none">
-    // https://w3c.github.io/mediacapture-fromelement/
-
-    partial interface HTMLMediaElement {
-        MediaStream captureStream();
-    };
-
-    partial interface HTMLCanvasElement {
-        MediaStream captureStream(optional double frameRequestRate);
-    };
-
-    interface CanvasCaptureMediaStreamTrack : MediaStreamTrack {
-        readonly attribute HTMLCanvasElement canvas;
-        void requestFrame();
-    };
-
-  </pre>
   <script>
     'use strict';
 
     promise_test(async () => {
-      const main = await fetch('/interfaces/mediacapture-main.idl').then(r => r.text());
-      const html = await fetch('/interfaces/html.idl').then(r => r.text());
-      const dom = await fetch('/interfaces/dom.idl').then(r => r.text());
+      const srcs = ['mediacapture-fromelement', 'mediacapture-main', 'html', 'dom'];
+      const [idl, main, html, dom] = await Promise.all(
+        srcs.map(i => fetch(`/interfaces/${i}.idl`).then(r => r.text())));
 
       var idl_array = new IdlArray();
-      idl_array.add_idls(document.getElementById("idl").textContent);
+      idl_array.add_idls(idl);
       idl_array.add_dependency_idls(main);
       idl_array.add_dependency_idls(html);
       idl_array.add_dependency_idls(dom);