As I pointed out on the CL, the platform is shared process wide, so you might end up in a situation where one Isolate holds on to a lot of external memory and another isolate starts to GC a lot without affecting that number

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/f314b3cdb9766eb1ba6efe1243ac89b3d0295f63

commit f314b3cdb9766eb1ba6efe1243ac89b3d0295f63
Author: hpayer <hpayer@chromium.org>
Date: Tue May 30 13:18:17 2017

[api] Add an API function to set the GetExternallyAllocatedMemoryInBytesCallback.

BUG=chromium:727569

Review-Url: https://codereview.chromium.org/2910203002
Cr-Commit-Position: refs/heads/master@{#45599}

[modify] https://crrev.com/f314b3cdb9766eb1ba6efe1243ac89b3d0295f63/include/v8.h
[modify] https://crrev.com/f314b3cdb9766eb1ba6efe1243ac89b3d0295f63/src/api.cc
[modify] https://crrev.com/f314b3cdb9766eb1ba6efe1243ac89b3d0295f63/src/heap/heap.cc
[modify] https://crrev.com/f314b3cdb9766eb1ba6efe1243ac89b3d0295f63/src/heap/heap.h