and also move embedder fields from Isolate to IsolateData.

The external memory counter fields are temporarily moved to IsolateData in
order to avoid unexpected Node JS bot failures which happen if the fields
are left in the Heap class.

Bug: v8:8182
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I9d58f235c0ce40e110f595addd03b80b3617aa77
Reviewed-on: https://chromium-review.googlesource.com/c/1278793
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57037}