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}
bool ContainerNode::checkParserAcceptChild(const Node& newChild) const
{
    if (!isDocumentNode())
        return true;
    return toDocument(*this).canAcceptChild(newChild, nullptr, IGNORE_EXCEPTION);
}

bool ContainerNode::checkParserAcceptChild(const Node& newChild) const
{
    if (!isDocumentNode())
        return true;
    return toDocument(*this).canAcceptChild(newChild, nullptr, IGNORE_EXCEPTION);
}
