This adds a runtime flag that controls whether the WASM object is
exposed to JavaScript. While currently guarded by the V8_WASM build-time
flag, after landing in V8 the build-time flag will be removed, leaving
only this new runtime flag.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1507623002

Cr-Commit-Position: refs/heads/master@{#32657}