This will require an equivalent sexpr-wasm change.

See:
https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#high-level-structure

R=titzer@chromium.org, binji@chronium.org

Review URL: https://codereview.chromium.org/1765843002

Cr-Commit-Position: refs/heads/master@{#34668}