This CL is preparation for moving the contents of the WasmContext
directly into the WasmInstanceObject. The moved fields are all untagged
pointers to C++ memory or untagged sizes which will be used in
generated machine code for WASM. They are not currently used, but
they are all set to kHeapObjectTag to make sure they are not
interpreted as tagged by the GC, using a custom object descriptor.

R=mstarzinger@chromium.org
CC=clemensh@chromium.org

Bug: v8:7424
Change-Id: Ie5d5161df32564dcac74c6ff659f1a38ddca3cb0
Reviewed-on: https://chromium-review.googlesource.com/961065
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51932}