Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

include/gpu/effects/GrExtractAlphaFragmentProcessor.h

-/*
- * Copyright 2015 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef GrExtractAlphaFragmentProcessor_DEFINED
-#define GrExtractAlphaFragmentProcessor_DEFINED
-
-#include "GrFragmentProcessor.h"
-
-/** This processor extracts the incoming color's alpha, ignores r, g, and b, and feeds
-    the replicated alpha to it's inner processor. */
-class GrExtractAlphaFragmentProcessor : public GrFragmentProcessor {
-public:
-    static GrFragmentProcessor* Create(const GrFragmentProcessor* processor) {
-        if (!processor) {
-            return nullptr;
-        }
-        return SkNEW_ARGS(GrExtractAlphaFragmentProcessor, (processor));
-    }
-
-    ~GrExtractAlphaFragmentProcessor() override {}
-
-    const char* name() const override { return "Extract Alpha"; }
-
-private:
-    GrExtractAlphaFragmentProcessor(const GrFragmentProcessor* processor) {
-        this->initClassID<GrExtractAlphaFragmentProcessor>();
-        this->registerChildProcessor(processor);
-    }
-
-    GrGLFragmentProcessor* onCreateGLInstance() const override;
-
-    void onGetGLProcessorKey(const GrGLSLCaps&, GrProcessorKeyBuilder*) const override;
-
-    bool onIsEqual(const GrFragmentProcessor&) const override;
-
-    void onComputeInvariantOutput(GrInvariantOutput* inout) const override;
-
-    GR_DECLARE_FRAGMENT_PROCESSOR_TEST;
-
-    typedef GrFragmentProcessor INHERITED;
-};
-
-#endif