Move settings_dropdown_menu styling to settings_shared_css.

chrome/browser/resources/settings/search_page/search_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-dropdown-menu/paper-dropdown-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-listbox/paper-listbox.html">
 <link rel="import" href="/search_engines_page/search_engines_page.html">
 <link rel="import" href="/search_engines_page/search_engines_browser_proxy.html">
 <link rel="import" href="/settings_page/settings_animated_pages.html">
 <link rel="import" href="/settings_page/settings_subpage.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-search-page">
   <template>
     <style include="settings-shared">
-      paper-dropdown-menu {
-        /* TODO(dschuyler): share styling with settings-dropdown-menu */
-        --iron-icon-fill-color: var(--paper-grey-600);
-      }
-
-      paper-listbox div {
-        align-items: center;
-        color: var(--paper-grey-800);
-        display: flex;
-        font-size: inherit;
-        min-height: 48px;
-        padding: 0 16px;
-      }
-
-      paper-listbox div.iron-selected {
-        font-weight: bold;
-      }
-
-      paper-listbox div:focus {
-        background-color: var(--paper-grey-300);
-        outline: none;
-      }
     </style>
     <settings-animated-pages id="pages" current-route="{{currentRoute}}"
         section="search">
       <neon-animatable id="main">
         <div class="settings-box first">
           <p class="start">$i18n{searchExplanation}</p>
           <paper-dropdown-menu>
             <paper-listbox class="dropdown-content"
                 selected="[[getSelectedSearchEngineIndex_(searchEngines_)]]"
                 on-iron-select="onIronSelect_">
               <template is="dom-repeat" items="[[searchEngines_]]">
                 <!--TODO(dpapad): Use <button class="paper-item">..</button>
                     once it lands in paper-item-shared-styles.html-->
-                <div role="option">[[item.name]]</div>
+                <div class="dropdown-item" role="option">[[item.name]]</div>
               </template>
             </paper-listbox>
           </paper-dropdown-menu>
         </div>
         <div class="settings-box">
           <paper-button on-tap="onManageSearchEnginesTap_"
               class="primary-button">
             $i18n{searchEnginesManage}
           </paper-button>
         </div>
       </neon-animatable>
       <template is="dom-if" name="search-engines">
         <settings-subpage page-title="$i18n{searchEnginesManage}">
           <settings-search-engines-page></settings-search-engines-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="search_page.js"></script>
 </dom-module>