This flag allows to share a single WasmEngine among all Isolates within
the same process. It will ultimately allow to share the WasmCode objects
associated with modules that are transferred via structured cloning.

R=clemensh@chromium.org
TEST=mjsunit/wasm/worker-module
BUG=v8:7424

Change-Id: I70d852d319b2a80bd02e0a2a838dcdfa071df6e1
Reviewed-on: https://chromium-review.googlesource.com/1138213
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54678}