title: Issue while styling a cell on onCellValueChanged in ag-grid
body: <p>I am trying to style a cell with a particular background color if the 'oldValue' and 'newValue' are not equal in the 'onCellValueChanged' handler. </p>
  
  <pre><code>var gridOptions = {
      columnDefs: columnDefs,
      rowData: rowData,
      onCellValueChanged: function(params) {
        if(params.oldValue !== params.newValue) {
          params.colDef.cellStyle = function(params) {
          return { backgroundColor: 'green'}; 
        }
      }
    }
   };
  </code></pre>
  
  <p>When I am editing the cell value to a new value, I don't see the style change being applied on tabbing out or clicking any other cell. Rather, I have to click back on the edited cell again and click outside after that to see the style being applied. I am not sure what is happening, can someone guide me here. I am also adding the <a href="https://plnkr.co/edit/T6QvkiD7CGhcnB10CI10?p=preview" rel="nofollow noreferrer">demo</a></p>