Consistently check if a block can handle pagination strut propagation.

https://codereview.chromium.org/1360753002 got it right for inline child
layout, but did nothing for block child layout.

BUG=329421
R=jchaffraix@chromium.org,leviw@chromium.org

Review URL: https://codereview.chromium.org/1387553002

Cr-Commit-Position: refs/heads/master@{#352429}
LayoutRect LayoutBlockFlow::selectionRectForPaintInvalidation(const LayoutBoxModelObject* paintInvalidationContainer) const
{
    LayoutRect rect = selectionGapRectsForPaintInvalidation(paintInvalidationContainer);
    if (paintInvalidationContainer->layer()->groupedMapping())
        PaintLayer::mapRectToPaintBackingCoordinates(paintInvalidationContainer, rect);
    return rect;
}

LayoutRect LayoutBlockFlow::selectionRectForPaintInvalidation(const LayoutBoxModelObject* paintInvalidationContainer) const
{
    LayoutRect rect = selectionGapRectsForPaintInvalidation(paintInvalidationContainer);
    if (paintInvalidationContainer->layer()->groupedMapping())
        PaintLayer::mapRectToPaintBackingCoordinates(paintInvalidationContainer, rect);
    return rect;
}
