SkRWBuffer for thread-safe 'stream' sharing

WIP
- Can accumulate (write) data in one thread, and share snapshots of it in other threads
  ... e.g. network accumulates image data, and periodically we want to decode/draw it
- If this sort of thing sticks, should we promote SkData to have the same generality as
  SkRBuffer?

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1106113002
4 files changed