share helper for flagging when added to raster cache

Rename to clarify that it is a "raster" cache we're talking about.

clean up -- motivated by looking to make the purge of a staleID synchronous
Bug: skia:
Change-Id: I87493690dd5a4f2ebd002e2635ebd0e44fe27320
Reviewed-on: https://skia-review.googlesource.com/142325
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
7 files changed