Beyond setting the total cache limits this method enables clients to
request to purge a specific number of bytes, as well as specify their
preference to purge scratch resources over resources of other types.
Change-Id: I9259d5544d34251575d77eebe599388f213ff3ce
Reviewed-on: https://skia-review.googlesource.com/17987
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>
