Bug 858292 - Preserve drawing buffer when using SkiaGL r=gwright

--HG--
extra : rebase_source : a3dc5c425039c77b536812be5517f27d3f7b86f1
Bug 865298 - Allow multiple touch interceptors on LayerView. r=kats
Bug 865298 - Add an 'active area' for margin exposing. r=kats

This stops margins from being exposed unless the drag was started in an area
of the viewport that's near said margin. The margins will always be exposed
when reaching the edge of the page.
Bug 865298 - Don't allow margins to scroll on small pages. r=kats

Bug 858969 regressed this behaviour. Margins that are smaller than the screen
size should have pinned margins.
Bug 850783 - Fix the dynamic toolbar causing clipping of the scrollbar. r=kats
Bug 861705 - Check for ParallelArray being disabled in various tests r=sstangl