DetachArrayBuffer makes incorrect assumptions about the state of the ArrayBuffer. It assumes that that the ArrayBuffer is internal to wasm unless guard pages are enabled, this is not the case as the ArrayBuffer can be externalized outside of wasm, in this case through gin.

BUG=chromium:700384

Review-Url: https://codereview.chromium.org/2754153002
Cr-Commit-Position: refs/heads/master@{#43880}