NewSpace::UpdateInlineAllocationLimit was computing the limit slighly
differently. Make old space and new space more consistent. The way
new space does it makes more sense as, logically, the step starts from
beyond the current object being allocated (size_in_bytes).

This behaviour change in preperation for a subsequent CL that refactors
a common SpaceWithLinearArea::ComputeLimit.

NewSpace: :UpdateInlineAllocationLimit and PagedSpace::ComputeLimit into
Change-Id: Ibe918d46dccf8e80ed35c770b3c365c3970d07ea
Reviewed-on: https://chromium-review.googlesource.com/815277Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#49974}