The WebAssembly JS API specification [1] covers the JS-visible side-effects
of executing a grow_memory operation and states that a successful
grow operation should always detach any prior array buffer.

[1] https://github.com/WebAssembly/spec/blob/master/document/js-api/index.bs

R=mstarzinger@chromium.org,gdeepti@chromium.org

Bug: 
Change-Id: Ib9232e01209ba546c0bba1c9408c92da60ff6d92
Reviewed-on: https://chromium-review.googlesource.com/860011Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50627}