Instead, create it when needed and pass it down to the actual
compilation.
This saves memory by making the WasmCompilationUnit smaller and will
eventually allow us to implement the trap handler fallback correctly by
using an updated ModuleEnv in background compilation and tier up.

R=mstarzinger@chromium.org

Bug: v8:5277, v8:8343
Change-Id: I0dc3a37fb88e54eb4822dc99d58ff024f4b2a367
Reviewed-on: https://chromium-review.googlesource.com/c/1293953
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56896}