Reland "Add unit test for nested runtime blending"

This is a reland of commit 5e19781093fb3f902c80e603b3c3194c3296df0b

Original change's description:
> Add unit test for nested runtime blending
>
> Change-Id: Iedc07205a7334ddf5edbe4d1fac5f19e1665d0b0
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/711258
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>

Change-Id: If77b983075371fccbc0acd27af3c516e63e501b2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/710364
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed