When cloning into an SkPicture, clear its data.
When cloning into an SkPicture, clear its data.

Prevents a memory leak if the caller clones into an SkPicture
that already has a playback or record.

Review URL: https://codereview.appspot.com/6813081

csulmone/skia,csulmone/skia,csulmone/skia,csulmone/skia