StyleAdjuster has the following:

  // After performing the display mutation, check table rows. We do not honor
  // position: relative table rows or cells.  This has been established for
  // position: relative in CSS2.1 (and caused a crash in containingBlock() on
  // some sites).
  if ((style.display() == EDisplay::TableHeaderGroup ||
       style.display() == EDisplay::TableRowGroup ||
       style.display() == EDisplay::TableFooterGroup ||
       style.display() == EDisplay::TableRow) &&
      style.position() == RelativePosition)
    style.setPosition(StaticPosition);

However, support for cells was added back in 2011, in 23fab7d4abe23de5b2855c4d0313467f2c8e4779 (svn://svn.chromium.org/blink/trunk@81049). We should fix up the comment, at least to avoid confusing future me.