Use input FPs as the base layer for processor tests.

Previously, this test layered its paints by calling
`addColorFragmentProcessor` multiple times. We intend to remove support
for multiple color FPs on a GrPaint in the near future, so we use input
fragment processors instead to generate the same result.

Change-Id: I33e82ce0067183189e69b2af0fe0c228d1d60f14
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303479
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
1 file changed