Consolidate PixelRef flattables with the standard impl

Consolidate PixelRef flattables with the standard impl

The flatten method on these functions can no longer be
const as SkFlattenables declaration is not const and 
would result in the const methods only being called when
the reference to the object was const.
Committed: https://code.google.com/p/skia/source/detail?r=3533

[DerekS, 2012-03-28 20:28:48]

gcl created a new issue when I updated my change. The original issue is
http://codereview.appspot.com/5939053/

[caryclark1, 2012-03-28 20:41:04]

lgtm