This reverts commit 57bf0bfe.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [wasm] Merge the WasmContext into WasmInstanceObject
> 
> 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}

TBR=mstarzinger@chromium.org,titzer@chromium.org,ahaas@chromium.org,clemensh@chromium.org

Change-Id: I653e27b46dbc43ad773eda4292d521a508f42d79
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7424
Reviewed-on: https://chromium-review.googlesource.com/995418Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52364}