Make this a member method of WasmTableObject, because it is pretty
coupled with that object anyways, and is always being called together
with WasmTableObject::Set or WasmTableObject::AddDispatchTable.
It also simplifies another refactoring: https://crrev.com/c/866733

R=titzer@chromium.org

Change-Id: I53392fb9cf21f2e45c2a144d180e9b3614657094
Reviewed-on: https://chromium-review.googlesource.com/866933Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50605}