ok, rate limit forking
ok, rate limit forking

On Macs the user process limit is low, in the hundreds, and it's a
little annoying to mysteriously find other programs fail to start
because ok is greedily slurping up all the available process capacity.

On Linux the user process limit is up in the ten-thousands, so I don't
expect this to matter there, even on Android.  I haven't seen any
significant speed difference imposing a default ncpus limit.

Change-Id: Id284723808074441710c0436acbd75ab1c6bbbb3
Reviewed-on: https://skia-review.googlesource.com/41840
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,rubenvb/skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,google/skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc