We want to move to a world where there's no Isolate::Current but we
always knows which isolate we're in. There's no way we can teach this
info to the C++ allocator.

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1128023005

Cr-Commit-Position: refs/heads/master@{#28414}