- Move into v8::internal::heap namespace
- Remove card marking ifdefs

Bug: 
Change-Id: Ifd5e5d96c6ab0fea85a3646e5b307583eb13e2c3
Reviewed-on: https://chromium-review.googlesource.com/612066Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47305}