Send SkPicture deletion message lazily.

Send SkPicture deletion message lazily.

If no one has read the picture's unique ID, there's no point invalidating it.

This is the same trick we pull with SkPixelRefs.

Before:
  26M	1	1.49µs	1.6µs	1.77µs	6.25µs	42%	picture_overhead_draw
  13M	32	742ns	749ns	756ns	823ns	2%	picture_overhead_nodraw

After:
  26M	1	1.27µs	1.33µs	1.49µs	5.51µs	45%	picture_overhead_draw
  14M	43	677ns	680ns	681ns	701ns	1%	picture_overhead_nodraw

BUG=skia:

Review URL: https://codereview.chromium.org/1061283002