Add SkSourceGammaTreatment enum so we know how to create mips

src/core/SkColorShader.h

Index: src/core/SkColorShader.h
diff --git a/src/core/SkColorShader.h b/src/core/SkColorShader.h
index f7e609498b8a96347b6bf44e6f2e08a11f164bb8..e2ee8a81bd238724dedff9374127656188bfcac9 100644
--- a/src/core/SkColorShader.h
+++ b/src/core/SkColorShader.h
@@ -50,7 +50,8 @@ public:
 
 #if SK_SUPPORT_GPU
     const GrFragmentProcessor* asFragmentProcessor(GrContext*, const SkMatrix& viewM,
-                                                   const SkMatrix*, SkFilterQuality) const override;
+                                                   const SkMatrix*, SkFilterQuality,
+                                                   bool gammaCorrect) const override;
 #endif
 
     SK_TO_STRING_OVERRIDE()
@@ -104,7 +105,8 @@ public:
 
 #if SK_SUPPORT_GPU
     const GrFragmentProcessor* asFragmentProcessor(GrContext*, const SkMatrix& viewM,
-                                                   const SkMatrix*, SkFilterQuality) const override;
+                                                   const SkMatrix*, SkFilterQuality,
+                                                   bool gammaCorrect) const override;
 #endif
 
     SK_TO_STRING_OVERRIDE()