Bug 828088 - Rework richgrid and richgriditem bindings to use css columns for down-then-across grids. Hardening downloads and tests. r=fryn

--HG--
extra : amend_source : 745c26b0287819fd4d002e9ca193c13618bbca0a
Bug 873222 - check unit-type of font-synthesis value before use. r=dbaron