Revert "[realbox] Makes all text in realbox impossible to select"

This reverts commit 3bfb8a37.

Reason for revert: crbug.com/1156059

Fixed: 1156059

Original change's description:
> [realbox] Makes all text in realbox impossible to select
>
> crrev.com/c/2566505 made all text in realbox dropdown unselectable.
>
> This CL extends that to the realbox placeholder as well. Note that
> non-placeholder text in the realbox input remains selectable after
> this change.
>
> Fixed: 1154576
> Change-Id: I80d13521680a165c56e2a7d98eff82c885ff3da4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572680
> Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
> Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
> Auto-Submit: Moe Ahmadi <mahmadi@chromium.org>
> Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#833512}

TBR=mahmadi@chromium.org,tiborg@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ic569859f08ba8ee7bc0d82cd4671088f3f4348d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577600Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834366}

chrome/browser/resources/new_tab_page/realbox.html

@@ -4,7 +4,6 @@
     border-radius: calc(0.5 * var(--ntp-realbox-height));
     box-shadow: 0 1px 6px 0 rgba(32, 33, 36, .28);
     height: var(--ntp-realbox-height);
-    user-select: none;
   }
 
   :host([matches-are-visible]) {

chrome/browser/resources/new_tab_page/realbox_dropdown.html

 <style>
   :host {
     font-size: 16px;
+    user-select: none;
   }
 
   #selector {