This patch ensures that an object returned by AllocateRaw is marked
black if black allocation starts during the object allocation.

This fixes the following issue:
1) Generated code requests allocation of size N for folded allocation.
2) Runtime gets a free list node at address A of size N+M and sets up
   a linear allocation area with top = A+N and limit = A+N+M.
3) Runtime invokes the allocation observer that starts incremental marking
   and start black allocation. The area [A+N, A+N+M) is marked black.
4) Runtime returns a white object at address A as the allocation result.
5) Generated code moves the top pointer to A and does bump pointer
   allocations of white objects from A to A+N+M.
6) Object allocated new A+N can have the impossible marbit pattern.

Bug: chromium:694255
Change-Id: I09ceebc97a510fa5fe4ff20706bc46a99f8b7cf4
Reviewed-on: https://chromium-review.googlesource.com/638338
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48005}