DevTools: DataGrid: do not use collapsed borders.

Actually, there is already no duplicated borders
to collapse.

Calculation of collapsed borders takes noticeable amount of time.

With separate borders layout doesn't do additional calculations.

Also, result is more predictable.

BUG=

Review URL: https://codereview.chromium.org/391733002

git-svn-id: svn://svn.chromium.org/blink/trunk@178151 bbb929c8-8fbe-4397-9dbb-9b2b20218538