Test: Don't add scrollbar width to intrinsic width of fixed-width grid items

The scroll bar width should only be added if the width of the grid item is
not fixed.

This test coverage has been inspired from the webkit patch for flexboxes at
https://bugs.webkit.org/attachment.cgi?id=182187&action=diff 

BUG=395788

Review URL: https://codereview.chromium.org/532203002

git-svn-id: svn://svn.chromium.org/blink/trunk@181590 bbb929c8-8fbe-4397-9dbb-9b2b20218538