The generated code for JSToWasm wrappers only depends on the signature
of the exported function. Hence, we can reuse the generated code and
just patch the reference to the called wasm code.

For the unity-wasm benchmark, we reach a hit rate of 98.07% for this
cache, and only 395 instead of 20471 wrappers are compiled. This brings
down instantiation time from 2.9s to 1.6s on a MBP.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2705993002
Cr-Commit-Position: refs/heads/master@{#43322}