Move settings_dropdown_menu styling to settings_shared_css.

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_root_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;
         font-size: 100%;
         font-weight: 500;
         margin: 0;
         min-height: var(--settings-row-min-height);
       }
 
       iron-icon[icon='cr:check'],
       iron-icon[icon='settings:done'] {
        --iron-icon-fill-color: var(--google-green-500);
       }
 
       paper-button {
         margin: 0;
       }
 
       paper-button[toggles][active] {
         background-color: var(--paper-grey-300);
       }
 
+      paper-dropdown-menu {
+        --iron-icon-fill-color: var(--paper-grey-600);
+        --paper-font-subhead: {
+          font-size: inherit;
+        };
+        --paper-input-container-underline: {
+          background: var(--paper-grey-300);
+        };
+      }
+
+      paper-dropdown-menu .dropdown-item {
+        align-items: center;
+        color: var(--paper-grey-800);
+        display: flex;
+        font-size: inherit;
+        min-height: 48px;
+        padding: 0 16px;
+      }
+
+      paper-dropdown-menu .dropdown-item.iron-selected {
+        font-weight: bold;
+      }
+
+      paper-dropdown-menu .dropdown-item:focus {
+        background-color: var(--paper-grey-300);
+        outline: none;
+      }
+
       span ~ a {
         -webkit-margin-start: 4px;
       }
 
       [is='action-link'],
       [is='action-link']:active,
       [is='action-link']:hover,
       [is='action-link']:visited,
       paper-button.primary-button,
       paper-button.tertiary-button {
@@ skipping 253 matching lines @@
 
       .favicon-image {
         background-repeat: no-repeat;
         background-size: contain;
         height: 16px;
         width: 16px;
       }
     </style>
   </template>
 </dom-module>