The number of worker threads in the default platform is currently capped
at 8. At runtime, it's also capped by the number of actual cores
available.
Since d8 is often used for benchmarking (both internally and
externally), this CL slightly increases the maximum to 16. At least wasm
compilation is known to scale beyond 8 threads. If this increase does
not cause any problems on bots, we might increase further in the future.

R=mlippautz@chromium.org

Change-Id: I04e138c51d7dc26c38b9384c8ea67a883c976050
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182454
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67586}