Fix layer cache memory leak

The issue was that, with caching disabled, the layer cache code was removing layers outside of a purge (i.e., in unlock) but not correctly cleaning up the GrPictureInfo objects (as is done in purgePlot).

Review URL: https://codereview.chromium.org/703643002
Separate JSON functions from DMWriteTask.

Add JsonWriter, which handles Json output from DM, in preparation for
adding json output for tests. This change should not affect behavior.

BUG=skia:2454

Review URL: https://codereview.chromium.org/702513003