Remove WebFrame::canHaveSecureChild

To simplify the public API, ServiceWorkerNetworkProvider can do the
parent walk itself.

Follow-up to https://crrev.com/ad1850962644e19.

BUG=607543

Review-Url: https://codereview.chromium.org/2082493002
Cr-Commit-Position: refs/heads/master@{#400896}
Page* Frame::page() const
{
    if (m_host)
        return &m_host->page();
    return nullptr;
}

Page* Frame::page() const
{
    if (m_host)
        return &m_host->page();
    return nullptr;
}
