[CSS Grid Layout] Prevent issues with checkLayout() in grid items

[CSS Grid Layout] Prevent issues with checkLayout() in grid items
https://bugs.webkit.org/show_bug.cgi?id=130770

Reviewed by Sergio Villar Senin.

When we use checkLayout() on a grid item, the results (PASS or FAIL
messages) are added as new auto-placed items in the grid. Which might
cause some unexpected behavior in the tests.

This patch modifies the related layout tests to use a different
container (usually the parent node of the grid) for the test results.
Avoiding any kind of interference with the grid code.

* fast/css-grid-layout/grid-item-addition-auto-placement-update.html:
  Use parent node of grid when checking grid items. Check layout of the
  whole grid instead of grid item.
* fast/css-grid-layout/grid-item-removal-auto-placement-update.html:
  Check layout of the whole grid instead of grid item.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166290 268f45cc-cd09-0410-ab3c-d52691b4dbfc