If a WebAssembly function is exported, its js-to-wasm wrapper has a
field which contains a reference to the WebAssembly function.
Originally this reference was an index into the export table, which
then contains an index into the function table, which then contains
the metadata of the WebAssembly function.

With this CL we use the index into the function table directly as
the reference to the WebAssembly function.

TEST=mjsunit/wasm/test-import-export-wrapper
R=rossberg@chromium.org, mtrofin@chromium.org
CC=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2472103002
Cr-Commit-Position: refs/heads/master@{#40729}