This changes JS-to-Wasm wrappers to no longer embed a WeakCell with the
associated instance into the code, but load the instance object from the
passed {WasmExportedFunction} object instead.

R=titzer@chromium.org
BUG=v8:7424

Change-Id: I5403f882912eb23e760fabe70207440648754a69
Reviewed-on: https://chromium-review.googlesource.com/1028053
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53057}