This adds two fields to the HeapStatistics struct:
- number_of_native_contexts,
- number_of_detached_contexts.

Bug: chromium:793789
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: If6942a97fd22a9e70781eed2aa286aba4c0e7f70
Reviewed-on: https://chromium-review.googlesource.com/819730
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50007}