This changes WebAssembly runtime stub compilation to no longer require
an Isolate parameter. Such stubs are shared between Isolates and hence
must be independent of the concrete Isolate anyways. Also we will soon
generate such stubs from background tasks, where we should not pass an
Isolate parameter either.

R=clemensh@chromium.org
CC=jkummerow@chromium.org

Change-Id: Id9de46cae3fb77946d745a4aeadccac50f6e8350
Reviewed-on: https://chromium-review.googlesource.com/c/1405039Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58710}