Bug 1050159. Apply correct dirty rect when AddCanvasBackgroundColorItem creates an nsDisplaySolidColor. r=mattwoodrow

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extra : rebase_source : a762a87108b7b7a36664a5590b43fbd28b4842d1
Bug 1062100. Part 1: Remove ItemCoversScrollableArea. r=mattwoodrow
This isn't really part of this bug but it simplifies related code.
Bug 1062100. Part 2: Assign clip rects to non-async-scrollable scrolled layers, and hide clipped layers if some non-moving layer covers their entire clip rect. r=mattwoodrow
Bug 1061045. Remove dirtyRect save/restore from Enter/LeavePresShell since they're not needed anymore. r=tn
Bug 1063052. NS_RUNTIMEABORT if a builtin stylesheet fails to load. r=heycam