Update Plyr custom styles for menu container

Specifically, set a maximum height and added vertical scrolling
to address an issue related to Plyr's menu height.

Improve the overall usability and visual appearance of the menu in video player.

youtube/static/modules/plyr/custom_plyr.css

@@ -29,3 +29,11 @@ e.g. Firefox playback speed options */
 .plyr__preview-thumb {
     bottom: 100%;
 }
+
+.plyr__menu__container [role="menu"],
+.plyr__menu__container [role="menucaptions"] {
+    /* Set vertical scroll */
+    /* issue https://github.com/sampotts/plyr/issues/1420 */
+    max-height: 320px;
+    overflow-y: auto;
+}