MD Settings: settings-dropdown-menu, restore "Enter" key behavior.

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;
@@ skipping 20 matching lines @@
         --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;

[michaelpg, 2016/07/12 23:44:22]

Have you checked if there's anything else in paper-item-shared-styles.html that
should be copied here? There's a lot of styles packed in there.

[dpapad, 2016/07/13 01:01:07]

I checked. The only thing that I see missing is the a dropdown-item[disabled]
style. But I also can't find any such cases in our UI. We tend to not show such
options at all, instead of showing them but have them disabled.

+        background: none;
+        border: none;
         color: var(--paper-grey-800);
         display: flex;
-        font-size: inherit;
+        font: inherit;
         min-height: 48px;
         padding: 0 16px;
+        width: 100%;
       }
 
       paper-dropdown-menu .dropdown-item.iron-selected {
         font-weight: bold;
       }
 
       paper-dropdown-menu .dropdown-item:focus {
         background-color: var(--paper-grey-300);
         outline: none;
       }
@@ skipping 277 matching lines @@
 
       .favicon-image {
         background-repeat: no-repeat;
         background-size: contain;
         height: 16px;
         width: 16px;
       }
     </style>
   </template>
 </dom-module>