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}
void Frame::detachChildren()
{
    typedef HeapVector<Member<Frame>> FrameVector;
    FrameVector childrenToDetach;
    childrenToDetach.reserveCapacity(tree().childCount());
    for (Frame* child = tree().firstChild(); child; child = child->tree().nextSibling())
        childrenToDetach.append(child);
    for (const auto& child : childrenToDetach)
        child->detach(FrameDetachType::Remove);
}

void Frame::detachChildren()
{
    typedef HeapVector<Member<Frame>> FrameVector;
    FrameVector childrenToDetach;
    childrenToDetach.reserveCapacity(tree().childCount());
    for (Frame* child = tree().firstChild(); child; child = child->tree().nextSibling())
        childrenToDetach.append(child);
    for (const auto& child : childrenToDetach)
        child->detach(FrameDetachType::Remove);
}
