Remove backing scale factor from widget

[classilla]

This has been low-hanging fruit for a long time and it's time to harvest it since we don't have to worry about merging changes anymore.

• Remove GetBackingScaleFactor(nsView) and nsChildView::BackingScaleFactor(), and all callers
• RoundsWidgetCoordinatesTo() should always return 1. Also, in dom/events/EventStateManager.cpp, simplify GetWindowClientRectCenter(nsIWidget* aWidget), and in view/nsView.cpp, simplify LayoutDeviceIntRect nsView::CalcWidgetBounds(nsWindowType aType)
• Remove backing scale calculations from CocoaPointsToDevPixels and DevPixelsToCocoaPoints, making them 1.0f, and make them explicitly inline. Consider simplifying callers since it's an unused argument at that point.
• Cleanup remnants "search for BackingScale"

[classilla]

Done. Nice win on the iMac G4 in particular.