ement. The child element with the class `dropdown-content` will be
used as the "content" element.

The `paper-menu-button` is sensitive to its content's `iron-select` events. If
the "content" element triggers an `iron-select` event, the `paper-menu-button`
will close automatically.

Example:

    <paper-menu-button>
      <paper-icon-button icon="menu" class="dropdown-trigger"></paper-icon-button>
      <paper-menu class="dropdown-content">
        <paper-item>Share</paper-item>
        <paper-item>Settings