Move external memory counters out of IsolateData back into Heap.
The class ExternalMemoryAccounting now stores all counters and is
responsible for updates. This change will allow turning counters into
atomic variables.

Bug: v8:10315
Change-Id: I2abeda298d3cfcc630fd04ca78a3d6d703e3b419
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2346647Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69356}