Unexport private SkShader classes

Alternative to https://skia-review.googlesource.com/c/18025/: these
subclasses are private, we shouldn't need to SK_API-tag them.

Change-Id: I94fc38e76ee4468132e41ab16f21ae6ad61fe1f2
Reviewed-on: https://skia-review.googlesource.com/18029
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed