This also makes sure 6.0 and 6.1 have the same constants exposed
in v8.h.

Bug: v8:6592

R=bmeurer@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I3afc0e5e4495594c76229555aab148ac78388f80
Reviewed-on: https://chromium-review.googlesource.com/569618
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46642}