replace CSXformSteps::Required() in SkShader
replace CSXformSteps::Required() in SkShader

This call is working perfectly fine as far as I reckon, but it's subtly
error-prone to not explicitly mention the alpha types when making this
kind of decision.  It just so happens that Requires() used premul/premul
just as we do here, so this should be a no-op.

Change-Id: I96850cc4109a6ac3c46bb7bbb467a30628ee8d44
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257658
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

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