Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

src/core/SkShader.cpp

Index: src/core/SkShader.cpp
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index 2a1c28e0cf8684ebcc57035080f12b9f5f15eb67..0ff025c1a91fe05c0f2c81ba3f1c439cb773d3ca 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -216,6 +216,17 @@ const GrFragmentProcessor* SkShader::asFragmentProcessor(GrContext*, const SkMat
     return nullptr;
 }
 
+#if SK_SUPPORT_GPU
+
+#include "effects/GrXfermodeFragmentProcessor.h"
+
+const GrFragmentProcessor* SkShader::MulFragmentProcesorOuputByInputAlpha(
+    const GrFragmentProcessor* fp) {
+    return GrXfermodeFragmentProcessor::CreateFromDstProcessor(fp, SkXfermode::kDstIn_Mode);
+}
+
+#endif
+
 SkShader* SkShader::refAsALocalMatrixShader(SkMatrix*) const {
     return nullptr;
 }