This is a first set of test cases for sharing an {WasmEngine} and the
contained {WasmCode} between multiple Isolates. Currently this can only
be done using internal API methods on the Isolate, an external API that
is usable by embedders does not exist yet.

R=clemensh@chromium.org
TEST=cctest/test-wasm-shared-engine
BUG=v8:7424

Change-Id: I35541a76b5aceec4519e3a46e6a9ef4d01cad22b
Reviewed-on: https://chromium-review.googlesource.com/1126382Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54248}