This makes the fact that {WasmSharedModuleData} is shared across
instances explicit by hanging this {shared} reference off the module
object instead of the instance-specific {WasmCompiledModule} object.

R=titzer@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I99bf3d855d6283bdc48373f0f8e2df1990905d3f
Reviewed-on: https://chromium-review.googlesource.com/1051909
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53163}