Add alternate SkPicture::clone

This adds an alternate version of SkPicture::clone for two reasons:

1) Chromium uses the SkPicture copy constructor to unpack the pictures from the old-style clone interface (and I would like to remove the copy ctor)

2) This is part of the long term plan to wean Chrome off of cloning. Once pictures are thread safe we will switch the new SkPicture::clone call to just return 'this'. From there it is a small step to removing clone entirely.

Note that the two versions of clone() is temporary. Once this is landed (and rolled) I will land a Chrome-side patch to remove their use of the old interface (Use new SkPicture::clone interface - https://codereview.chromium.org/380323002/)

R=mtklein@google.com, reed@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/381193002

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