The primary purpose of this is to untangle a circular dependency
objects.h -> handles.h -> objects.h. Most compilation units only
need message-template.h, without the rest of messages.h.
Bonus: change the enum to an enum class for improved type safety.

Bug: v8:3770
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I8102c55197a450811de2588a68a08e7f99ea6b9e
Reviewed-on: https://chromium-review.googlesource.com/c/1272193
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56583}