Fix an optimisation in ContainerNode::notifyNodeInsertedInternal

R=tkent@chromium.org
BUG=544020

Review URL: https://codereview.chromium.org/1420653003

Cr-Commit-Position: refs/heads/master@{#355240}
unsigned ContainerNode::countChildren() const
{
    unsigned count = 0;
    Node* n;
    for (n = firstChild(); n; n = n->nextSibling())
        count++;
    return count;
}

unsigned ContainerNode::countChildren() const
{
    unsigned count = 0;
    Node* n;
    for (n = firstChild(); n; n = n->nextSibling())
        count++;
    return count;
}
