Don't render <select> separators on Android

The <select> listbox popup supports <hr> elements which get turned into
"separators" as exposed by the mojo interface. On some platforms like
MacOS, these separators get rendered as horizontal lines between
selectable options in the listbox popup. On Android these are getting
rendered as selectable options with no text instead of actual
separators. Every option already has separators rendered in between
them, so we should ignore these on Android.

Bug: 1494927
Change-Id: Ieed9bdbe1338cb251e60961bec80c418f667d8d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4968656
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1218577}