move compose-shader into base-class as factory

next steps:
1. this lands
2. update android to call the new factory
3. move SkComposeShader.h into private

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1686803003

Review URL: https://codereview.chromium.org/1686803003
2 files changed