Bug 1076447. When building the display list for scrollbars and we have a display port just use the whole scrollbar rect as the dirty rect. r=mattwoodrow

When we have a display port the dirty rect for root scroll frames is going to be the display port already when it is passed in. The display port can contain a somewhat arbitrary area of the scroll frame, there is no reason is has to include the scrollbars, which are laid out at the desktop scrollport (and adjusted as necessary in the compositor).

As well as BuildDisplayListForChild, we need the dirty rect to be set for the creation of the nsDisplayWrapList or nsDisplayOwnLayer in AppendToTop.