emitChild() takes an outputColor instead of generating one

src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrExtractAlphaFragmentProcessor.h"
 #include "gl/GrGLFragmentProcessor.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 
 class GLExtractAlphaFragmentProcessor : public GrGLFragmentProcessor {
 public:
     GLExtractAlphaFragmentProcessor() {}
 
     void emitCode(EmitArgs& args) override {
         GrGLFragmentBuilder* fsBuilder = args.fBuilder->getFragmentShaderBuilder();
         fsBuilder->codeAppendf("vec4 alpha4 = %s.aaaa;", args.fInputColor);
-        SkString output;
-        this->emitChild(0, "alpha4", &output, args);
-        fsBuilder->codeAppendf("%s = %s;", args.fOutputColor, output.c_str());
+        this->emitChild(0, "alpha4", args.fOutputColor, args);
     }
 
 private:
     typedef GrGLFragmentProcessor INHERITED;
 };
 
 GrGLFragmentProcessor* GrExtractAlphaFragmentProcessor::onCreateGLInstance() const {
     return SkNEW(GLExtractAlphaFragmentProcessor);
 }
 
 void GrExtractAlphaFragmentProcessor::onGetGLProcessorKey(const GrGLSLCaps&,
                                                           GrProcessorKeyBuilder*) const {
 }
 
 bool GrExtractAlphaFragmentProcessor::onIsEqual(const GrFragmentProcessor&) const { return true; }
 
 void GrExtractAlphaFragmentProcessor::onComputeInvariantOutput(GrInvariantOutput* inout) const {
     if (inout->validFlags() & kA_GrColorComponentFlag) {
         GrColor color = GrColorPackA4(GrColorUnpackA(inout->color()));
         inout->setToOther(kRGBA_GrColorComponentFlags, color,
                           GrInvariantOutput::kWill_ReadInput);
     } else {
         inout->setToUnknown(GrInvariantOutput::kWill_ReadInput);
     }
     this->childProcessor(0).computeInvariantOutput(inout);
 }