Re-enable an ASSERT in RenderTableSection::paint.

The ASSERT triggered on table-section-node-at-point-crash.html,
which why it was reverted. Testing locally, it doesn't seem to
trigger anymore so let's re-enable it.

See http://trac.webkit.org/changeset/124641 for the story.

Review URL: https://chromiumcodereview.appspot.com/23604047

git-svn-id: svn://svn.chromium.org/blink/trunk@157488 bbb929c8-8fbe-4397-9dbb-9b2b20218538