Add SkSourceGammaTreatment enum so we know how to create mips

src/core/SkShader.cpp

Index: src/core/SkShader.cpp
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index 1b729ea9a851f45ec42f5e370e86e38584832897..2ec3b0b6613d0ecea798458bf1ae138bfe3a3463 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -221,7 +221,8 @@ SkShader::GradientType SkShader::asAGradient(GradientInfo* info) const {
 }
 
 const GrFragmentProcessor* SkShader::asFragmentProcessor(GrContext*, const SkMatrix&,
-                                                         const SkMatrix*, SkFilterQuality)  const {
+                                                         const SkMatrix*, SkFilterQuality,
+                                                         SkSourceGammaTreatment)  const {
     return nullptr;
 }