Currently, EnumSet cannot be used with scoped enums (declared as "enum
class" or "enum struct"), as these cannot be implicitly casted to their
underlying integral type. This CL changes this by adding a static cast.
Plus drive-by fixes.

R=mstarzinger@chromium.org

Change-Id: I3aa2ef78e896b2734f1e5695237efc8a1130703c
Reviewed-on: https://chromium-review.googlesource.com/598789
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47120}