This separates the fast-non-asan path from the asan path so it can be inlined.
Additionally avoid updating allocation_size_ on each Zone::New call.
Inlining Zone::New actually reduces binary size by 50kb...

Change-Id: Ie5d58638284e5a1a5e0198c24080b0f600d79092
Reviewed-on: https://chromium-review.googlesource.com/c/1288641Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56780}