add gpu impl for shadermaskfilter

adds MulChildAlphaByInput
renames MulOutputByInputAlpha --> MulChildByInputAlpha

Bug: skia:7500
Change-Id: Ic0615d4d23a887fbee510901ed77a36f98a1b11d
Reviewed-on: https://skia-review.googlesource.com/102440
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 080d993..73e5f11 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -1094,7 +1094,7 @@
     if (GrPixelConfigIsAlphaOnly(config)) {
         fp = GrFragmentProcessor::MakeInputPremulAndMulByOutput(std::move(fp));
     } else {
-        fp = GrFragmentProcessor::MulOutputByInputAlpha(std::move(fp));
+        fp = GrFragmentProcessor::MulChildByInputAlpha(std::move(fp));
     }
 
     GrPaint grPaint;