Add SkSourceGammaTreatment enum so we know how to create mips

src/image/SkImage_Base.h

Index: src/image/SkImage_Base.h
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index 947af0e0d178629c8d1e7ce4a138ea305824ec3f..c01d9af63528efb18900ad0fa9a3bd55af3bdda1 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -49,7 +49,8 @@ public:
     }
 
     // Caller must call unref when they are done.
-    virtual GrTexture* asTextureRef(GrContext*, const GrTextureParams&) const = 0;
+    virtual GrTexture* asTextureRef(GrContext*, const GrTextureParams&,
+                                    SkSourceGammaTreatment) const = 0;
 
     virtual sk_sp<SkImage> onMakeSubset(const SkIRect&) const = 0;