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

src/core/SkImageCacherator.h

Index: src/core/SkImageCacherator.h
diff --git a/src/core/SkImageCacherator.h b/src/core/SkImageCacherator.h
index 42b93f62349e3fab744bbe16b7e5a3e370e74a81..6b000668dd83fa9cce786ba821a6caadb5629ccc 100644
--- a/src/core/SkImageCacherator.h
+++ b/src/core/SkImageCacherator.h
@@ -54,8 +54,11 @@ public:
     /**
      *  If the underlying src naturally is represented by an encoded blob (in SkData), this returns
      *  a ref to that data. If not, it returns null.
+     *
+     *  If a GrContext is specified, then the caller is only interested in gpu-specific encoded
+     *  formats, so others (e.g. PNG) can just return nullptr.
      */
-    SkData* refEncoded();
+    SkData* refEncoded(GrContext*);
 
     // Only return true if the generate has already been cached.
     bool lockAsBitmapOnlyIfAlreadyCached(SkBitmap*);