This allows this enumeration to be used from nsIPresShell.h without introducing
a circular dependency.
Its new home in layout/base/ScrollTypes.h, included as mozilla/ScrollTypes.h.
Others similar enums can be added to that file if desired.
This patch also makes ScrollMode an enum class (as it's no longer nested
inside a class) and switches its enumerators to the |eName| naming convention.
Differential Revision: https://phabricator.services.mozilla.com/D24796
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extra : moz-landing-system : lando
573a6eaafa