Add SkMiniData.

This is a bit like a limited SkData, geared to store really tiny byte strings.

This is not hooked up anywhere beyond the new unit test.  I did experimentally
plumb it into SkRecord for drawPosTextH: just over 40% of drawPosTextH calls in
our repo can fit into ShortData.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/573323002
5 files changed