LibWeb/HTML: Implement TextTrackList IDL interface

Author: https://github.com/jamierocks
Commit: ecad28657a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/447
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/TextTrackList.idl

@@ -0,0 +1,13 @@
+#import <DOM/EventTarget.idl>
+
+// https://html.spec.whatwg.org/multipage/media.html#texttracklist
+[Exposed=Window]
+interface TextTrackList : EventTarget {
+    readonly attribute unsigned long length;
+    getter TextTrack (unsigned long index);
+    TextTrack? getTrackById(DOMString id);
+
+    attribute EventHandler onchange;
+    attribute EventHandler onaddtrack;
+    attribute EventHandler onremovetrack;
+};