This makes the assumption about new-space allocation in the CSA more clear.
Additionally AllocateInNewSpace asserts that the allocation will fit in the
new-space in a debug build.

Change-Id: Ica5e7e12656dcdaa2c739b3d300fdcbaeb2355a2
Reviewed-on: https://chromium-review.googlesource.com/448043Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43557}