Use toHTMLTableCellElement as much as possible.

Also, we unify NDEBUG versions and !NDEBUG versions of toHTMLTableCellElement
because we have release+assertion configuration.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23443006

git-svn-id: svn://svn.chromium.org/blink/trunk@156773 bbb929c8-8fbe-4397-9dbb-9b2b20218538