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

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 321dcaaae7caa0ef76ae947c2dd71bc2f2d7bd85..4689abb3b78d5f7bdb1a3e08dad95e1040c1e8ac 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -51,7 +51,8 @@ public:
 
     virtual SkImage* onNewSubset(const SkIRect&) const = 0;
 
-    virtual SkData* onRefEncoded() const { return nullptr; }
+    // If a ctx is specified, then only gpu-specific formats are requested.
+    virtual SkData* onRefEncoded(GrContext*) const { return nullptr; }
 
     virtual bool onAsLegacyBitmap(SkBitmap*, LegacyBitmapMode) const;