make SkColorShader RP-only

Now that we're not creating them internally,
they should be rare-to-non-existent.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib8414a7096252e4c8c94d9ba447938fa467f0b26
Reviewed-on: https://skia-review.googlesource.com/c/168880
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed