title: matTooltipClass not applying css
body: <p>I am attempting to apply some css changes to <code>mat-tooltip</code> from angular material 2 and found in the documentation a <code>matTooltipClass</code> that can then be selected in the css file to make changes. However, I am not able to get it working. </p>
  
  <p>component.html :</p>
  
  <pre><code>  &lt;mat-cell 
      *matCellDef="let productInfo" 
      matTooltip="{{productInfo.description}}"
      matTooltipClass="tooltip"&gt;
       {{ productInfo.description}}
    &lt;/mat-cell&gt;
  </code></pre>
  
  <p>component.scss:</p>
  
  <pre><code>.tooltip {
  background-color: red;
  color: blue;
  font-size: 20px;
  }
  </code></pre>