A few notes:
1) Oilpan is a generic library, meaning that it can work with arbitrary
user types. The library is split in type-aware (include/) and
type-erased (src/) parts. The former comprises a lot of code that still
needs to be defended with dchecks;
2) Macros are prefixed with CPPGC_, so that they don't clash in the user
code with similar macros from other libraries;
3) The macros simply forward requests to V8 so that dcheck handlers can
be configured uniformly;
4) The CL doesn't contain CHECK_EQ and friends, but they can be added
later if needed.

Bug: chromium:1056170
Change-Id: I68e6f663247705233eaf030384164d81e53071e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2148774
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67129}