ASSERTION FAILED: child->isTableCell()

This ASSERTION Fails when the child is not a table cell.
Consider the scenario when there is a spelling mistake in defining
the role for a gridcell inside a grid.

Ex:
<div role="grid">
  <div role="row">
    <span role="columnheader">Word</span>
  </div>
  <div role="row">
      <span rolw="gridcell">ABCD</span>
  </div>
</div>

Also, this ASSERTION is not required because in the next line
'if' check has been made to check whether child is tablecell or not.

BUG=412735

Review URL: https://codereview.chromium.org/575743002

git-svn-id: svn://svn.chromium.org/blink/trunk@184139 bbb929c8-8fbe-4397-9dbb-9b2b20218538