MD Downloads: fix some a11y issues

chrome/browser/resources/md_downloads/toolbar.html

 <link rel="import" href="chrome://downloads/action_service.html">
 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-item/paper-item.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu-button/paper-menu-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar.html">
@@ skipping 76 matching lines @@
     <cr-toolbar id="toolbar" page-name="$i18n{title}"
         search-prompt="$i18n{search}" clear-label="$i18n{clearSearch}"
         spinner-active="{{spinnerActive}}" on-search-changed="onSearchChanged_">
       <div class="more-actions">
         <paper-menu-button id="more" horizontal-align="right"
             allow-outside-scroll>
           <paper-icon-button icon="cr:more-vert" title="$i18n{moreActions}"
                class="dropdown-trigger"></paper-icon-button>
           <paper-menu class="dropdown-content">
             <paper-item class="clear-all" on-tap="onClearAllTap_"
-                on-blur="onItemBlur_">
+                role="menuitem" on-blur="onItemBlur_">
               $i18n{clearAll}
             </paper-item>
             <paper-item on-tap="onOpenDownloadsFolderTap_"
-                on-blur="onItemBlur_">
+                role="menuitem" on-blur="onItemBlur_">
               $i18n{openDownloadsFolder}
             </paper-item>
           </paper-menu>
         </paper-menu-button>
       </div>
     </cr-toolbar>
   </template>
   <script src="chrome://downloads/toolbar.js"></script>
 </dom-module>