DevTools: hide padding rows by default in datagrid viewport.

BUG=
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/222253002

git-svn-id: svn://svn.chromium.org/blink/trunk@170663 bbb929c8-8fbe-4397-9dbb-9b2b20218538