heap-inl.h exposes the whole world, which is fine from other inline
files but not from regular headers.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I09ec67c6558682cb0d5181031bc39341a3f4c5bf
Reviewed-on: https://chromium-review.googlesource.com/643294Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47729}