Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

src/gpu/GrProcessor.cpp

Index: src/gpu/GrProcessor.cpp
diff --git a/src/gpu/GrProcessor.cpp b/src/gpu/GrProcessor.cpp
index 782b82ba5c34701faa21dfb67c43c38e22a3bf82..496e8ee218e8017d7a6b5878138a3b64d0e23991 100644
--- a/src/gpu/GrProcessor.cpp
+++ b/src/gpu/GrProcessor.cpp
@@ -241,6 +241,13 @@ bool GrFragmentProcessor::hasSameTransforms(const GrFragmentProcessor& that) con
     return true;
 }
 
+#include "effects/GrXfermodeFragmentProcessor.h"
+
+const GrFragmentProcessor* GrFragmentProcessor::MulOuputByInputAlpha(
+    const GrFragmentProcessor* fp) {
+    return GrXfermodeFragmentProcessor::CreateFromDstProcessor(fp, SkXfermode::kDstIn_Mode);
+}
+
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 // Initial static variable from GrXPFactory