[graphite] Add task dumping

* Add a new debugging utility for Graphite to dump the task list at
`Recorder::snap()`.

* This feature is disabled by default and can be enabled in debug builds
by setting the GN argument `skia_dump_tasks=true`.

Change-Id: I5f3fb9c25100e417a038e591f6cd5c9a4d1363bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1054636
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
19 files changed