Introudce toCDATASection() functions, and use them.

Many DOM node classes have such cast functions.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23600033

git-svn-id: svn://svn.chromium.org/blink/trunk@157592 bbb929c8-8fbe-4397-9dbb-9b2b20218538