To bring V8 into line with the proposed design changes in:

 https://github.com/WebAssembly/design/pull/489

(This CL is forked from https://codereview.chromium.org/1634673002/.
That CL doesn't merge cleanly, and I can't update it.)

TBR=titzer@chromium.org
LOG=Y
BUG=chromium:575167

Review URL: https://codereview.chromium.org/1682443002

Cr-Commit-Position: refs/heads/master@{#33828}