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
String CharacterIterator::string(int numChars)
{
    Vector<UChar> result;
    result.reserveInitialCapacity(numChars);
    while (numChars > 0 && !atEnd()) {
        int runSize = min(numChars, length());
        result.append(characters(), runSize);
        numChars -= runSize;
        advance(runSize);
    }
    return String::adopt(result);
}

String CharacterIterator::string(int numChars)
{
    Vector<UChar> result;
    result.reserveInitialCapacity(numChars);
    while (numChars > 0 && !atEnd()) {
        int runSize = min(numChars, length());
        result.append(characters(), runSize);
        numChars -= runSize;
        advance(runSize);
    }
    return String::adopt(result);
}
