Just use standard C++ syntax to define structs and enums instead.

R=ahaas@chromium.org

Bug: v8:9183
Change-Id: Ibae1643bd1dc74267cdd14ec45a36fc65bf0ab4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1631410Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61889}