Mutable globals are now included in the wasm v1 spec.

Bug: v8:7625
Change-Id: Ib9b92d8348102f99a3b92820d0057b2c11a1e49a
Reviewed-on: https://chromium-review.googlesource.com/1095650
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53683}