This reverts commit 5648aad5.

Reason for revert: Compile error on mips:
https://build.chromium.org/p/client.v8.ports/builders/V8%20Mips%20-%20builder/builds/10732

Original change's description:
> [wasm] Don't store global handles in the interpreter
> 
> Storing global handles in the interpreter is dangerous, because the
> global handles are strong roots into the heap. The interpreter itself is
> referenced from the heap via a Managed. Hence the interpreter keeps the
> instance alive, while the instance keeps the Managed alive. So the GC
> will never collect them.
> 
> This CL refactors this to only store the handle to the instance object
> while executing in the interpreter, and clearing it when returning.
> It also removes the cache of import wrappers, as it should not be
> performance critical, but keeps lots of objects alive. If it turns out
> to be performance critical, we will have to reintroduce such a cache
> stored in the WasmDebugInfo object.
> 
> R=​titzer@chromium.org
> CC=ahaas@chromium.org
> 
> Bug: chromium:610330
> Change-Id: I54b489dadc16685887c0c1a98da6fd0df5ad7cbb
> Reviewed-on: https://chromium-review.googlesource.com/567058
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46629}

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

Change-Id: Ifadfb885f937f37bb3eab4732a97f20ff40c2583
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:610330
Reviewed-on: https://chromium-review.googlesource.com/569962Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46630}