Bug 990631. When processing a native window move for a popup compare the new position with the actual position we expect to popup to be at. r=enndeakin a=lsblakk

The ScreenPosition() (which looks at mScreenX/YPos) doesn't actually hold the screen position for anchored popups.
Bug 989897. In order to properly compute root composition size view the root composition size first as layer pixels in the target layer, then convert to css pixels. r=kats a=lsblakk

Since our goal is to use the root composition size to bound the layer size of the display port of the child layer this makes sense.