DevTools: DataGrid: tune filler rows CSS class names.

Removed column name from filler row td.
Separated top & bottom filler row classes.

BUG=

Review URL: https://codereview.chromium.org/426413004

git-svn-id: svn://svn.chromium.org/blink/trunk@179313 bbb929c8-8fbe-4397-9dbb-9b2b20218538