Disable frame navigations during DocumentLoader detach in FrameLoader::startLoad

BUG=613266

Review-Url: https://codereview.chromium.org/2006033002
Cr-Commit-Position: refs/heads/master@{#396241}
HTMLAnchorElement* HTMLAnchorElement::create(Document& document)
{
    return new HTMLAnchorElement(aTag, document);
}

HTMLAnchorElement* HTMLAnchorElement::create(Document& document)
{
    return new HTMLAnchorElement(aTag, document);
}
