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
UChar TextIterator::characterAt(unsigned index) const
{
    ASSERT_WITH_SECURITY_IMPLICATION(index < static_cast<unsigned>(length()));
    if (!(index < static_cast<unsigned>(length())))
        return 0;

    if (!m_textCharacters)
        return string()[startOffset() + index];

    return m_textCharacters[index];
}

UChar TextIterator::characterAt(unsigned index) const
{
    ASSERT_WITH_SECURITY_IMPLICATION(index < static_cast<unsigned>(length()));
    if (!(index < static_cast<unsigned>(length())))
        return 0;

    if (!m_textCharacters)
        return string()[startOffset() + index];

    return m_textCharacters[index];
}
