This makes sure that allocations of {HeapNumber} objects happening in
the JS-to-Wasm and Wasm-to-JS wrappers are ordered with respect to
changes of the {trap_handler::IsThreadInWasm} predicate. Otherwise the
compiler can (and will) move the allocations across changes of this
predicate and cause safety checks to fire.

R=clemensh@chromium.org

Change-Id: I5366ec0c184929fbd5b60c827d9908fb6ca1d91a
Reviewed-on: https://chromium-review.googlesource.com/1042399
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52979}