This fixes a long-standing TODO to only make a copy of a module's
wire bytes if the input is a SharedArrayBuffer and also fixes the
concurrent-modification bug for synchronous validation.

R=clemensh@chromium.org
BUG=chromium:794091

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I8d2f20a9aeedbc306434853f8f6cfc070a24cf97
Reviewed-on: https://chromium-review.googlesource.com/856559
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50472}