This change makes lifetime management of WasmCode much simpler.
By using the WasmInstanceObject as the context for WASM code execution,
including the pointer to the memory base and indirect function tables,
this keeps the instance alive when WASM code is on the stack, since
the instance object is passed as a parameter and spilled onto the stack.
This is in preparation of sharing the code between instances and
isolates.

Bug: v8:7424

Change-Id: Ic2e4b7bcc2feb20001d0553a615a8a9dff36317e
Reviewed-on: https://chromium-review.googlesource.com/958520
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52361}