Add SkSourceGammaTreatment enum so we know how to create mips

src/effects/gradients/SkLinearGradient.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLinearGradient_DEFINED
 #define SkLinearGradient_DEFINED
 
@@ skipping 42 matching lines @@
                                                    float invDx, const float dither[2]);
 
         typedef SkGradientShaderBase::GradientShaderBaseContext INHERITED;
     };
 
     GradientType asAGradient(GradientInfo* info) const override;
 #if SK_SUPPORT_GPU
     const GrFragmentProcessor* asFragmentProcessor(GrContext*,
                                                    const SkMatrix& viewM,
                                                    const SkMatrix*,
-                                                   SkFilterQuality) const override;
+                                                   SkFilterQuality,
+                                                   SkSourceGammaTreatment) const override;
 #endif
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkLinearGradient)
 
 protected:
     SkLinearGradient(SkReadBuffer& buffer);
     void flatten(SkWriteBuffer& buffer) const override;
     size_t onContextSize(const ContextRec&) const override;
     Context* onCreateContext(const ContextRec&, void* storage) const override;
 
 private:
     class LinearGradient4fContext;
 
     friend class SkGradientShader;
     typedef SkGradientShaderBase INHERITED;
     const SkPoint fStart;
     const SkPoint fEnd;
 };
 
 #endif