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

R=mstarzinger@chromium.org

Change-Id: Ia35a3ce91a8f6135767fa764e185cde8bbc889f4
Reviewed-on: https://chromium-review.googlesource.com/997932
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52436}