Have SkPicture inherit from SkRefCnt.

This re-enables adoption tracking for SkPictures in Blink,
which should be green now that crrev.com/1136123011 has landed.

BUG=skia:3847

Review URL: https://codereview.chromium.org/1145153002
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index 995db4c..edbe748 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -28,10 +28,7 @@
     An SkPicture records drawing commands made to a canvas to be played back at a later time.
     This base class handles serialization and a few other miscellany.
 */
-// TODO(mtklein): logically this could be : public SkRefCnt, but that exposes
-// some ref-count adoption bugs in Blink.  Keeping this using SkNVRefCnt
-// happens to stifle them for now.  skia:3847
-class SK_API SkPicture : public SkNVRefCnt<SkPicture> {
+class SK_API SkPicture : public SkRefCnt {
 public:
     virtual ~SkPicture();