Reland "Replace pooling mechanism with GrMemoryPool."

This is a reland of 67e1cf4b1d81185051bdaaaab4a1d88723e57358

The iOS 8 code path now compiles normally

Original change's description:
> Replace pooling mechanism with GrMemoryPool.
>
> This change is a wash for tests that could fit inside the previous
> hard-coded pool (512 nodes) and appears to be a 5% improvement for
> sksl_large. Larger programs would hypothetically show an even more
> significant improvement.
>
> When SK_SUPPORT_GPU is disabled, we disable pooling entirely and fall
> back to the system allocator. This is necessary because SkSL can exist
> without Ganesh (such as in the wasm+CanvasKit build).
>
> Nanobench: http://screen/4xJEzdGducRxGeq
>
> Change-Id: I71dc702a84ab5c163673e35ec651003d7d45dacd
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330219
> Commit-Queue: John Stiles <johnstiles@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>

Change-Id: Iced330084f1ed8997e19bbee585422cb89e1c6b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330404
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed