MD Settings: Fix Search tooltip arrow obscuring letters

chrome/browser/resources/settings/settings_shared_css.html

 <link rel="import" href="chrome://resources/cr_elements/shared_style_css.html">
 <link rel="import" href="/settings_vars_css.html">
 
 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style include="cr-shared-style">
       /* Included here so we don't have to include "iron-positioning" in every
        * stylesheet. See crbug.com/498405. */
       [hidden] {
@@ skipping 413 matching lines @@
       /* Provides the arrow which points at the anchor element. */
       .search-bubble-innards::after {
         -webkit-transform: rotate(-45deg);
         background-color: var(--paper-yellow-500);
         content: '';
         height: 10px;
         left: 55px;
         position: absolute;
         top: -5px;
         width: 10px;
+        z-index: -1;
       }
 
       /* Turns the arrow direction downwards, when the bubble is placed above
        * the anchor element */
       .search-bubble-innards.above::after {
         -webkit-transform: rotate(-135deg);
         bottom: -5px;
         top: auto;
       }
     </style>
   </template>
 </dom-module>