In the current implementation, WasmInstanceWrapper is allocated after the imports for the instance are processed, and before the InstanceFinalizer callback is associated with the instance. This raises the possibility of triggering a gc in the middle of the instantiate flow which is incorrect.

BUG=5707

R=titzer@chromium.org, petermarshall@chromium.org

Review-Url: https://codereview.chromium.org/2544273002
Cr-Commit-Position: refs/heads/master@{#41464}