Back off from SkPicture API hardening a bit

Sigh - Chromium still relies on the old clone functionality

TBR=reed@google.com

Review URL: https://codereview.chromium.org/371273006
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index 8e54b1d..a346606 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -241,7 +241,8 @@
 
     mutable uint32_t      fUniqueID;
 
-    SkAutoTDelete<const SkPictureData> fData;
+    // TODO: make SkPictureData const when clone method goes away
+    SkAutoTDelete<SkPictureData> fData;
     int                   fWidth, fHeight;
     mutable SkAutoTUnref<const AccelData> fAccelData;