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
static inline RenderText* firstRenderTextInFirstLetter(RenderObject* firstLetter)
{
    if (!firstLetter)
        return 0;

    for (RenderObject* current = firstLetter->firstChild(); current; current = current->nextSibling()) {
        if (current->isText())
            return toRenderText(current);
    }
    return 0;
}

static inline RenderText* firstRenderTextInFirstLetter(RenderObject* firstLetter)
{
    if (!firstLetter)
        return 0;

    for (RenderObject* current = firstLetter->firstChild(); current; current = current->nextSibling()) {
        if (current->isText())
            return toRenderText(current);
    }
    return 0;
}
