Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

include/gpu/GrFragmentProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrFragmentProcessor_DEFINED
 #define GrFragmentProcessor_DEFINED
 
 #include "GrProcessor.h"
 
 class GrCoordTransform;
 class GrGLSLCaps;
 class GrGLFragmentProcessor;
 class GrInvariantOutput;
 class GrProcessorKeyBuilder;
 
 /** Provides custom fragment shader code. Fragment processors receive an input color (vec4f) and
     produce an output color. They may reference textures and uniforms. They may use
     GrCoordTransforms to receive a transformation of the local coordinates that map from local space
     to the fragment being processed.
  */
 class GrFragmentProcessor : public GrProcessor {
 public:
+    /**
+    *  In many instances (e.g. SkShader::asFragmentProcessor() implementations) it is desirable to
+    *  only consider the input color's alpha. However, there is a competing desire to have reusable
+    *  GrFragmentProcessor subclasses that can be used in other scenarios where the entire input
+    *  color is considered. This function exists to filter the input color and pass it to a FP. It
+    *  does so by returning a parent FP that multiplies the passed in FPs output by the parent's
+    *  input alpha. The passed in FP will not receive an input color.
+    */
+    static const GrFragmentProcessor* MulOuputByInputAlpha(const GrFragmentProcessor*);
+
     GrFragmentProcessor()
         : INHERITED()
         , fUsesLocalCoords(false)
         , fNumTexturesExclChildren(0)
         , fNumTransformsExclChildren(0) {}
 
     ~GrFragmentProcessor() override;
 
     GrGLFragmentProcessor* createGLInstance() const;
 
@@ skipping 145 matching lines @@
     int                                          fNumTransformsExclChildren;
 
     // TODO: These must convert their processors to pending-execution refs when the parent is
     // converted (do this automatically in GrProgramElement?).
     SkTArray<const GrFragmentProcessor*, true>   fChildProcessors;
 
     typedef GrProcessor INHERITED;
 };
 
 #endif