Bug 982141 - Expose the logic used to decide whether a scrollable frame should be async scrollable. r=tn

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extra : rebase_source : 6ce26d8d54bbf75c79b10fd8a8dc449b0d417387
Bug 982141 - Move CalculateRootCompositionSize to nsLayoutUtils and clean it up a bit. r=tn

--HG--
extra : rebase_source : e5b29f38d38f88d6a214bdd1e2e31354018ccadd
Bug 982141 - Calculate frame metrics for a display port calculation. r=kats,tn

--HG--
extra : rebase_source : bbb27a00dfc190c9c78563b149ca10aa6bfb41d5
Bug 982141 - Factor out 'wantSubAPZC' into nsLayoutUtils. r=kats,tn

--HG--
extra : rebase_source : c23e9d3253991523f39d7437ab5f36172768813a
Bug 982141 - Introduce nsLayoutUtils::SetDisplayPortMargins, with an option to not repaint. r=tn

--HG--
extra : rebase_source : 95d6fb6992221c86800bc6322730729c083d75a3
Bug 982141 - Make sure the primary async-scrollable frame has a displayport set. r=tn

--HG--
extra : rebase_source : 6ceb20abb14f3e829ea06c3b3b911455cff9db23