take gr-context parameter to refEncoded, indicating a desire for only gpu-specific formats

src/image/SkImage.cpp

Index: src/image/SkImage.cpp
diff --git a/src/image/SkImage.cpp b/src/image/SkImage.cpp
index 338ab0913090da08bb1a030eaec9d20bea99c6c8..32844b474fe370d936fbfa4865555c7356bbada9 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -202,7 +202,8 @@ SkData* SkImage::encode(SkPixelSerializer* serializer) const {
 }
 
 SkData* SkImage::refEncoded() const {
-    return as_IB(this)->onRefEncoded();
+    GrContext* ctx = nullptr;   // should we allow the caller to pass in a ctx?
+    return as_IB(this)->onRefEncoded(ctx);
 }
 
 SkImage* SkImage::NewFromEncoded(SkData* encoded, const SkIRect* subset) {