Reland of SkShaderBase

Introduce a private base class (SkShaderBase), to hide
implementation details from the public interface (SkShader).

Change-Id: Ib1d76cde880bd51868b97408710f8bb38128e536
Reviewed-on: https://skia-review.googlesource.com/17925
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
61 files changed