Move logic of checking whether a frame needs to be flattened to RenderFrameBase.

[zalan]

So that flattening checking code can somewhat be shared between frameset and iframe. That would eliminate the need of early return and mimicking non-flattening layout case in RenderFrameBase::layoutWithFlattening(), which can get (and is) out of sync in case of iframe layouting.
See below.

RenderFrameBase::layoutWithFlattening(...)
{
...
    if (!width() || !height() || !childRoot) {
        updateWidgetPosition();
        if (childFrameView)
            childFrameView->layout();
        setNeedsLayout(false);
        return;
...
}

vs.

RenderIFrame::layout() 
{
...
    RenderPart::layout();

    m_overflow.clear();
    addVisualEffectOverflow();
    updateLayerTransform();
    setNeedsLayout(false);
...
}