Set user-select: none on the media-app sandboxed <iframe>.

If there are clicks in the window before the <iframe> loads, it's
possible for the <iframe> itself to become selected. This puts a gray
overlay over the window contents which is impossible to remove without
reloading.

Fixed: b/227678322
Change-Id: I2886963bf11f0f5f869dd5eb8929a97af180425b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3565218
Reviewed-by: Zain Afzal <zafzal@google.com>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/main@{#988370}

ash/webui/media_app_ui/resources/index.html

@@ -18,13 +18,14 @@
   /* Any changes to this file should also be copied to index_dark_light.html. */
 
   /*
-   * This is the <iframe> style set for sandboxed guests that use
-   * guest_view_iframe_container.js.
-   * TODO(crbug/996088): Remove the iframe styles if switched to <webview>.
+   * This is the <iframe> style set for sandboxed chrome-untrusted:// guests.
+   * `user-select: none` is applied here to avoid the iframe obtaining a grey
+   * overlay if it is clicked while loading (b/227678322).
    */
   iframe {
     border: 0;
     height: 100%;
+    user-select: none;
     width: 100%;
   }
 </style>