The current impl for SkFlattenable read/write buffers is
that they extend from SkReader32 and SkWriter32, but that
dependency must be abstract if we are to add any other
serialization format.
Review URL: https://codereview.appspot.com/5999045
git-svn-id: http://skia.googlecode.com/svn/trunk@3654 2bbb7eff-a529-9590-31e7-b0007b416f81