Hide SkPerlinNoiseShader impl details

Move the shader impl to a private class, leave SkPerlinNoiseShader as
a factory class only (similar to e.g. SkLightingShader).

Change-Id: Ic1180db8f5dfd3d8f6fba133c6bf6bbdfa4f97a4
Reviewed-on: https://skia-review.googlesource.com/17318
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed