Sanitize imports before we start the instance building process. This
avoids the possibility of exiting to JS while building instances,
and allowing JS to observe an inconsistent state of the wasm world -
e.g. incomplete specialization chains.

We now validate we never exit to JS during that process.

Bug: chromium:766260

Change-Id: I34930c8b70bdac16af464b3f62a2b6a38107acb3
Reviewed-on: https://chromium-review.googlesource.com/671480
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48074}