The WasmInstanceObject stores two new arrays:

- imported_mutable_globals_buffers_: a FixedArray of all the imported
  globals' array buffers.
- imported_mutable_globals: a calloc'd array of Addresses pointing to
  the mutable global in its array buffer.

When accessing the global, the generated code looks up the address in
imported_mutable_globals to find where to load/store.

Bug: v8:7625
Change-Id: I60844c21a788fce28f346455f10f2283d1c152e9
Reviewed-on: https://chromium-review.googlesource.com/1020602
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52794}