[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}

chrome/browser/resources/new_tab_page/realbox.html

@@ -4,6 +4,7 @@
     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 {