DevTools: DataGrid: introduce top filler row.

Currently datagrid have only bottom filler to pad tables to fit container.

Top filler is used by viewport datagrid subclasses.
But adding it make generic datagrid logic simpler.

BUG=

Review URL: https://codereview.chromium.org/384673003

git-svn-id: svn://svn.chromium.org/blink/trunk@178261 bbb929c8-8fbe-4397-9dbb-9b2b20218538