Bug 1109841 - Remove the C11 _Static_assert which checks a runtime condition; r=abr

clang-cl enables C11 and therefore complains on this code:
static_assert expression is not an integral constant expression.
Bug 1109705 - Fix more bad implicit constructors in layout; r=roc
Bug 1109729 - Fix more bad implicit constructors in mfbt; r=froydnj
Bug 1109699 - Fix more bad implicit constructors in gfx and image; r=jrmuizel
Bug 1109694 - Fix more bad implicit constructors in DOM; r=baku
Bug 1109697 - Fix more bad implicit constructors in netwerk; r=mcmanus