If one wasm instance imports an exported function of another instance,
we unwrap the js-to-wasm wrapper of the export and use the underlying
code object directly. However, the code object does not keep the wasm
instance alive. It is only connected via a WeakCell.
With this CL, we explicitly store a FixedArray of all wasm instances
from which we imported functions to keep them alive at least as long as
the instance which imports the code.

R=mtrofin@chromium.org, ahaas@chromium.org
BUG=chromium:734345

Change-Id: I8dcfc9a4ea2d791a62d8cb7255039e481c50bdfd
Reviewed-on: https://chromium-review.googlesource.com/539738Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46062}