The tests illustrate the use of v8::EmbedderHeapTracer.

Bug: v8:7176
Change-Id: Ic383c968691fddb0ec96d66cb33ee42b9c304a75
Reviewed-on: https://chromium-review.googlesource.com/811924
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49934}