Upgrade a TextIterator ASSERT to a RELEASE_ASSERT as a defensive measure.

      BUG=156930,177197
R=inferno@chromium.org

Review URL: https://codereview.chromium.org/15057010

git-svn-id: svn://svn.chromium.org/blink/trunk@150123 bbb929c8-8fbe-4397-9dbb-9b2b20218538