DevTools: Prepare for recursive viewport in datagrid

No behavioral changes, architecture only:
  - padding rows are moved back to the datagrid
  - children access are made through Node.allChildren()
  - minor cosmetic changes

BUG=255363
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/187703002

git-svn-id: svn://svn.chromium.org/blink/trunk@168480 bbb929c8-8fbe-4397-9dbb-9b2b20218538