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}
Document& Document::topDocument() const
{
    Document* doc = const_cast<Document*>(this);
    for (HTMLFrameOwnerElement* element = doc->localOwner(); element; element = doc->localOwner())
        doc = &element->document();

    DCHECK(doc);
    return *doc;
}

Document& Document::topDocument() const
{
    Document* doc = const_cast<Document*>(this);
    for (HTMLFrameOwnerElement* element = doc->localOwner(); element; element = doc->localOwner())
        doc = &element->document();

    DCHECK(doc);
    return *doc;
}
