Changed SkLightingShader API to take in a shader as color source

This CL's base is the CL for the same modification, but in the implementation side: https://codereview.chromium.org/2062703003/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2064153002

Review-Url: https://codereview.chromium.org/2064153002
6 files changed