This changes the WebAssembly serializer to take a full snapshot of the
code table before measuring and serializing the module. It allows other
threads (or other Isolates) to mutate the native module while the main
thread is serializing the module.

R=clemensh@chromium.org
BUG=v8:7424

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I64bc27600452880f37f4fbb2f40c77c79975358f
Reviewed-on: https://chromium-review.googlesource.com/1156596
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54838}