Bug 1042423 - Part 1: Clean up GetBackgroundClip and make it public. r=roc
Bug 1042423 - Part 2: Add DisplayItemClip constructor for rect + rounded rect. r=roc
Bug 1042423 - Part 3: Don't build inactive mask layers. r=roc
Bug 1042423 - Part 4: Clip background display items using DisplayItemClip. r=roc
Bug 1042423 - Part 5: Remove PaintBackgroundColor. r=roc