SkSemaphore: use SYNC_POLICY_LIFO on Mach.
SkSemaphore: use SYNC_POLICY_LIFO on Mach.

Turns out _LIFO does work as we expected, keeping N < M threads active when we
have M live threads but only N units of work to do at a time.  Also as we
expected _FIFO does round-robin through the threads.

Performance doesn't seem to be affected, but let's do it anyway.
If nothing else this makes profiles a little easier to read.

I don't see POSIX or Windows equivalents.

BUG=skia:

Review URL: https://codereview.chromium.org/1192433006

Jichao/skia,shahrzadmn/skia,google/skia,Hikari-no-Tenshi/android_external_skia,qrealka/skia-hc,shahrzadmn/skia,nvoron23/skia,pcwalton/skia,Jichao/skia,ominux/skia,qrealka/skia-hc,noselhq/skia,HalCanary/skia-hc,qrealka/skia-hc,todotodoo/skia,Jichao/skia,Hikari-no-Tenshi/android_external_skia,google/skia,vanish87/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,nvoron23/skia,shahrzadmn/skia,pcwalton/skia,HalCanary/skia-hc,todotodoo/skia,nvoron23/skia,pcwalton/skia,pcwalton/skia,todotodoo/skia,pcwalton/skia,tmpvar/skia.cc,noselhq/skia,todotodoo/skia,Hikari-no-Tenshi/android_external_skia,pcwalton/skia,Hikari-no-Tenshi/android_external_skia,vanish87/skia,Jichao/skia,noselhq/skia,HalCanary/skia-hc,vanish87/skia,qrealka/skia-hc,rubenvb/skia,ominux/skia,rubenvb/skia,shahrzadmn/skia,google/skia,vanish87/skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,google/skia,todotodoo/skia,nvoron23/skia,tmpvar/skia.cc,ominux/skia,Jichao/skia,nvoron23/skia,aosp-mirror/platform_external_skia,qrealka/skia-hc,tmpvar/skia.cc,shahrzadmn/skia,pcwalton/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,ominux/skia,qrealka/skia-hc,aosp-mirror/platform_external_skia,ominux/skia,noselhq/skia,ominux/skia,noselhq/skia,tmpvar/skia.cc,shahrzadmn/skia,aosp-mirror/platform_external_skia,vanish87/skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,pcwalton/skia,google/skia,noselhq/skia,ominux/skia,noselhq/skia,Jichao/skia,google/skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,aosp-mirror/platform_external_skia,ominux/skia,nvoron23/skia,google/skia,vanish87/skia,noselhq/skia,shahrzadmn/skia,shahrzadmn/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,noselhq/skia,todotodoo/skia,rubenvb/skia,todotodoo/skia,nvoron23/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,tmpvar/skia.cc,ominux/skia,tmpvar/skia.cc,tmpvar/skia.cc,rubenvb/skia,pcwalton/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Jichao/skia,tmpvar/skia.cc,HalCanary/skia-hc,qrealka/skia-hc,nvoron23/skia,rubenvb/skia,Jichao/skia,google/skia,todotodoo/skia,vanish87/skia,shahrzadmn/skia,todotodoo/skia,qrealka/skia-hc,Jichao/skia,vanish87/skia,rubenvb/skia,vanish87/skia