Add user-select:none to customizable select

Mouse-based text selection doesn't work in customizable select. Based on
this issue, we should at least set user-select:none in the UA
stylesheet: whatwg/html#10876

Bug: 385069402
Change-Id: I87cbfde4cfde67c7b1fe131a6070311034143bd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6111420
Commit-Queue: Joey Arhar <[email protected]>
Reviewed-by: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1402577}

Author: josepharhar

third_party/blink/renderer/core/html/resources/customizable_select.css

@@ -30,8 +30,9 @@ select:not(:-internal-list-box) {
   display: -internal-appearance-auto-base-select(inline-block, inline-flex);
   gap: -internal-appearance-auto-base-select(initial, 0.5em);
   border-radius: -internal-appearance-auto-base-select(0, 0.5em);
-
   align-items: -internal-appearance-auto-base-select(center, unset);
+  /* https://github.com/whatwg/html/issues/10876 */
+  user-select: -internal-appearance-auto-base-select(auto, none);
 }
 
 select:not(:-internal-list-box) > button:first-child {