This CL does same changes as
https://chromium-review.googlesource.com/c/540763/, but for async
compilation instead of for parallel compilation. The biggest difference
is that for async compilation I start background tasks again when half
of the memory is free again and not when all the memory is free again.

Original description:

It is possible that the foreground task is unable to clear the
scheduled unfinished work, eventually leading to an OOM.

We use either code_range on 64 bit, or the capacity of the code space,
as a heuristic for how much memory to use for compilation.

The change avoids blocking the background threads while we're over the
memory threshold. This is to avoid starving the GC.

R=mtrofin@chromium.org

Change-Id: I7399e2474f72f6727e6e50176dd7ba95cdcd3238
Reviewed-on: https://chromium-review.googlesource.com/543477
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46120}