This enables direct uses in e.g.
  unique_ptr<v8::EmbedderHeapTracer>

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I11f86c916dca5c31413866d9972178fccda7df9c
Reviewed-on: https://chromium-review.googlesource.com/1160538Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54883}