Port gradient unit tests over to new FPs

Previously, the GPU gradient tests were included in the SkXGradient
implementation and provided a processor factory that used the public
SkGradientShader::MakeX() end points to get at the final GrFragmentProcessor.

This factory approach is maintained as well, so there are no individual
tests for the specific colorizers. The tests are added to the layouts that
correspond to the public gradient constructor.

Bug: skia:
Change-Id: Iac74d8490efd0fb7395ad9fc0581906abb815742
Reviewed-on: https://skia-review.googlesource.com/151548
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
15 files changed