Add color gamut xform helpers to GrGLSLShaderBuilder

src/gpu/glsl/GrGLSLColorSpaceXformHelper.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLSLColorSpaceXformHelper_DEFINED
 #define GrGLSLColorSpaceXformHelper_DEFINED
 
+#include "GrColorSpaceXform.h"
 #include "GrGLSLUniformHandler.h"
 
-class GrColorSpaceXform;
-
 /**
  * Stack helper class to assist with using GrColorSpaceXform within an FP's emitCode function.
+ * This injects the uniform declaration, and stores the information needed to generate correct
+ * gamut-transformation shader code.
  */
 class GrGLSLColorSpaceXformHelper : public SkNoncopyable {
 public:
     GrGLSLColorSpaceXformHelper(GrGLSLUniformHandler* uniformHandler,
                                 GrColorSpaceXform* colorSpaceXform,
                                 GrGLSLProgramDataManager::UniformHandle* handle) {
         SkASSERT(uniformHandler && handle);
         if (colorSpaceXform) {
             *handle = uniformHandler->addUniform(kFragment_GrShaderFlag, kMat44f_GrSLType,
                                                  kDefault_GrSLPrecision, "ColorXform",
                                                  &fXformMatrix);
+            fAlphaType = colorSpaceXform->alphaType();
         } else {
             fXformMatrix = nullptr;
         }
     }
 
     const char* getXformMatrix() const { return fXformMatrix; }
+    SkAlphaType alphaType() const { return fAlphaType; }
 
 private:
     const char* fXformMatrix;
+    SkAlphaType fAlphaType;
 };
 
 #endif