Allow SkThreadPool to be non-borrowing

The GPU thread has privileged access to the GPU so its work can't be easily borrowed.

Change-Id: I1eae4c86ff1c36cc1248f74fc48d76b1c243f0b2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284764
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed