Start to rework DM JSON handling.

DM's striking off into its own JSON world.  This gets strawman implementations
in place for writing and reading a JSON file mapping test name to hashes.

For what it's worth, I basically want to change _all_ these pieces,
  - MD5 is slow and we can replace it with something faster,
  - JSON schema needs room to grow more data,
  - it'd be nice to hash once instead of twice when reading and writing,
  - this code wants lots of refactoring,
but this gives us a starting platform to work on these bits at our leisure.

E.x. file for now:

mtklein@mtklein ~/skia (dm)> cat good/dm.json
{
   "3x3bitmaprect_565" : "fc70d985fbfbe70e3a3c9dc626d4f5bc",
   "3x3bitmaprect_8888" : "df1591dde35907399734ea19feb76663",
   "3x3bitmaprect_gpu" : "df1591dde35907399734ea19feb76663",
   "aaclip_565" : "1862798689b838a7ab0dc0652b9ace3a",
   "aaclip_8888" : "47bb314329f0ce243f1d83fd583decb7",
   "aaclip_gpu" : "75f72412d0ef4815770202d297246e7d",
...

BUG=skia:
R=jcgregorio@google.com, stephana@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/546873002
4 files changed