title: Office Ui Fabric Dropdown doesn't expand to its content
body: <p>I am using Office UI Fabric React that comes with SPFx. It's version 6.214.0. I have a dropdown that looks like this:</p>
  <pre><code>&lt;Dropdown placeholder={strings.Position} label=&quot;&quot; options={positionOptions} onChange={(e, option) =&gt; setPosition(option.text)} styles = {dropdownStyles} className={styles.dropDown}/&gt;
  </code></pre>
  <p>The styles.dropDown has the following styles:</p>
  <pre><code>.dropDown{
    float: left; 
    margin-right: 15px; 
  }
  </code></pre>
  <p>The values are filled dynamically based on a web service. However the dropdown width doesn't fit the items inside.</p>
  <p><a href="https://i.stack.imgur.com/UjG0c.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UjG0c.png" alt="enter image description here" /></a></p>
  <p>If I remove the float left, all of them will take 100% of the width.</p>
  <p>How do we achieve this with UI Fabric React?</p>