Free unused scratch space from idle Pools.

GrBlockAllocators will hang onto an extra scratch node unless you ask
them to reset. When a Pool is detached from its thread, we know that
we're done with whatever work we needed to do, so it's a good time to
reset any scratch space and reclaim that memory.

Change-Id: I0ff2c4fad9f51a2f3dc911730abad77c06af8d57
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330276
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed