Downloads UI: Fix "clear all" cr-action-menu entry.

It was missed in r533917.

Bug: 809381,808182
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I4b7d574e2a1f6b3522547def1b2c6615e974143a
Reviewed-on: https://chromium-review.googlesource.com/904851
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534824}

chrome/browser/resources/md_downloads/toolbar.html

@@ -46,9 +46,10 @@
       </button>
     </cr-toolbar>
     <dialog is="cr-action-menu" id="moreActionsMenu">
-      <button class="dropdown-item clear-all" on-tap="onClearAllTap_">
+      <button slot="item" class="dropdown-item clear-all"
+          on-tap="onClearAllTap_">
         $i18n{clearAll}
-      </div>
+      </button>
       <button slot="item" class="dropdown-item"
           on-tap="onOpenDownloadsFolderTap_">
         $i18n{openDownloadsFolder}