Node.js is using AdjustAmountOfExternalAllocatedMemory to get
external memory from V8 [1]. In addition, they have a unittest
that verifies that AdjustAmountOfExternalAllocatedMemory returns
the correct value [2]. This CL proposes a new way to report
external memory through HeapStatistics.

[1]https://github.com/nodejs/node/blob/07cb69720bec251a1c450b3770db2136ac009778/src/node_process.cc#L187
[2]https://github.com/nodejs/node/blob/master/test/parallel/test-memory-usage.js

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ia58ed0bab1c1d4ee23672e1347b9a00b6705a43c
Reviewed-on: https://chromium-review.googlesource.com/1162156
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54919}