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}
bool FrameLoader::shouldEnforceStrictMixedContentChecking() const
{
    Frame* parentFrame = m_frame->tree().parent();
    if (!parentFrame)
        return false;

    return parentFrame->securityContext()->shouldEnforceStrictMixedContentChecking();
}

bool FrameLoader::shouldEnforceStrictMixedContentChecking() const
{
    Frame* parentFrame = m_frame->tree().parent();
    if (!parentFrame)
        return false;

    return parentFrame->securityContext()->shouldEnforceStrictMixedContentChecking();
}
