Redesign const-color unit test to leverage sk_gpu_test::test_ops.
Redesign const-color unit test to leverage sk_gpu_test::test_ops.

The previous implementation invoked SkPaintToGrPaint to create a
base-layer GrFragmentProcessor, and then manually poked an extra
GrFragmentProcessor onto the GrPaint's color processor stack. This
approach is not compatible with a world where a GrPaint is only allowed
a single GrFragmentProcessor.

Redesigned the test to use a different approach. We now manually create
the base-layer processor, and then create a second ConstColorProcessor
which uses the base layer as an input processor. This composite FP is
then drawn using `sk_gpu_test::test_ops`.

Change-Id: I8a2781549ba3d5aee85a520b9b34a70c0e66093a
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301986
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia