Add SkShaper::PurgeCaches.

Allows the user to signal that any global outstanding cached data should
be cleaned up to free resources.

Change-Id: I59d4bb2bbb4356920dea8caf912d9cb5f13014cf
Bug: skia:10763
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/360079
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Julia Lavrova <jlavrova@google.com>
5 files changed