Add toComment fuctions, and apply them as much as possible.

They have type checking assertions, and help to detect bad-cast bugs.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23816013

git-svn-id: svn://svn.chromium.org/blink/trunk@157584 bbb929c8-8fbe-4397-9dbb-9b2b20218538