Settings: Introduce selectable styling and apply to search engines

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">
       h2 {
         align-items: center;
         display: flex;
@@ skipping 49 matching lines @@
         text-align: start;
         width: 100%;
       }
 
       paper-dropdown-menu .dropdown-item.iron-selected {
         font-weight: bold;
       }
 
       iron-dropdown .dropdown-item:focus,
       paper-dropdown-menu .dropdown-item:focus {
-        background-color: var(--cr-focused-item-color);
-        outline: none;
+        @apply(--cr-selectable-focus);
       }
 
       iron-dropdown .dropdown-content {
         background-color: white;
         box-shadow: 0 2px 6px var(--paper-grey-500);
       }
 
       iron-dropdown .dropdown-content .dropdown-item,
       iron-dropdown .dropdown-content paper-item {
         @apply(--settings-actionable);
@@ skipping 283 matching lines @@
         content: '';
         height: 10px;
         position: absolute;
         right: -5px;
         top: 10px;
         width: 10px;
       }
     </style>
   </template>
 </dom-module>