Fix document actions menu with unresolved elements

Steps to reproduce:

Contribute a new document action as follows:

• Clone the nuxeo-web-ui repository from Github and set up dev environment.
• Edit elements/nuxeo-document-actions/nuxeo-my-tag-action.html:

  <dom-module id="nuxeo-my-tag-action">
    <template>
      <style>
      </style>
      <paper-icon-button id="tag" icon="icons:pets">
      </paper-icon-button>
      <paper-tooltip>[[i18n('documentMyTagAction.tooltip')]]</paper-tooltip>
    </template>
    <script>
      Polymer({
        is: 'nuxeo-my-tag-action',
        behaviors: [Nuxeo.I18nBehavior],
        properties: {
  
          /**
           * Input document.
           */
          document: {
            type: Object
          }
        }
      });
    </script>
  </dom-module>
  

• Reference it in elements/elements.html:

  diff --git i/elements/elements.html w/elements/elements.html
  index 1857ca16..230a4930 100644
  --- i/elements/elements.html
  +++ w/elements/elements.html
  @@ -252,6 +252,7 @@ limitations under the License.
   <link rel="import" href="../bower_components/nuxeo-ui-elements/actions/nuxeo-download-button.html">
   <link rel="import" href="nuxeo-document-actions/nuxeo-clipboard-toggle-button.html">
   <link rel="import" href="nuxeo-document-actions/nuxeo-document-edit-button.html">
  +<link rel="import" href="nuxeo-document-actions/nuxeo-my-tag-action.html">
   
   <link rel="import" href="../bower_components/nuxeo-ui-elements/nuxeo-format-behavior.html">
   <link rel="import" href="../bower_components/nuxeo-ui-elements/actions/nuxeo-preview-button.html">
  

• Add the needed DOCUMENT_ACTIONS slot contribution in elements/nuxeo-web-ui-bundle.html

  <nuxeo-slot-content name="myTagDocumentAction" slot="DOCUMENT_ACTIONS" order="0">
    <template>
     <nuxeo-my-tag-action document="[[document]]"></nuxeo-my-tag-action>
    </template>
  </nuxeo-slot-content>
  

• Refresh browser to view the new Document Action.
  => Scrollbars appear in order to display all the document actions (see screenshot).