This makes Engines (task execution strategies: serial, thread, fork)
pluggable just like most of the rest of ok. It removes the thread and
process limits, as I find myself rarely caring about what they are
exactly. Instead of limiting to num-cores, we just allow any number of
concurrent threads, and any number of concurrent child processes subject
to OS limitations.
Change-Id: Icef49d86818fe9a4b7380efb60e73e40bc2e6b73
Reviewed-on: https://skia-review.googlesource.com/27140
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
