Add an (optional) SkTaskGroup to GrContext

GrContextOptions has an SkExecutor field, allowing clients to supply a
thread pool. If present, the GrContext will create an SkTaskGroup that
can be used for internal threading work.

Bug: skia:
Change-Id: I8b01245515a21a83f9fe838caf0a01c9a26c0003
Reviewed-on: https://skia-review.googlesource.com/37580
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed