Add SkSourceGammaTreatment enum so we know how to create mips

src/image/SkImage_Gpu.h

Index: src/image/SkImage_Gpu.h
diff --git a/src/image/SkImage_Gpu.h b/src/image/SkImage_Gpu.h
index 612741ee7b7bc37c6f27a7a357d18464b59d3d8a..3cef9340323a4217d2b321cd4860106674bfb0ed 100644
--- a/src/image/SkImage_Gpu.h
+++ b/src/image/SkImage_Gpu.h
@@ -39,7 +39,8 @@ public:
     }
 
     bool getROPixels(SkBitmap*, CachingHint) const override;
-    GrTexture* asTextureRef(GrContext* ctx, const GrTextureParams& params) const override;
+    GrTexture* asTextureRef(GrContext* ctx, const GrTextureParams& params,
+                            SkSourceGammaTreatment) const override;
     sk_sp<SkImage> onMakeSubset(const SkIRect&) const override;
 
     GrTexture* peekTexture() const override { return fTexture; }