Bug 1018309 - Make the forced scrollbar layerization from bug 1016535 really only kick in for actual overlay scrollbars. r=roc

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extra : rebase_source : 525c4e242f28af85ad7c0f6cae8f2f4083b01d7d
Bug 1018464 - Add nsDisplayItem::IsInvisibleInRect API to expose holes in display items. r=roc

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extra : rebase_source : 9f362283113c0ce925ca81c3613ac8c6d19dfebe
Bug 1018464 - Make ContainerState::FindOpaqueBackgroundColorFor look through holes in display items. r=roc

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extra : rebase_source : 69d82381327e1114a6264d5ed360dd16aca39166