[realbox] Makes all text in realbox dropdown impossible to select

Previously only the realbox header text was unselectable. Making all
text in realbox dropdown unselectable is more consistent with omnibox.

Fixed: 1147142
Change-Id: Iade45c037e8e9208aba7ff0e708c985aa9ea97bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566505
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832575}

chrome/browser/resources/new_tab_page/realbox_dropdown.html

 <style>
   :host {
     font-size: 16px;
+    user-select: none;
   }
 
   #selector {
@@ -37,7 +38,6 @@
     padding-inline-end: 8px;
     text-overflow: ellipsis;
     text-transform: uppercase;
-    user-select: none;
     white-space: nowrap;
   }