Reduce ref counting in SkPictureRecorder.

This may be a small help to slimming paint:

  picture_overhead_draw 1.25us -> 1.22us    0.98x
  picture_overhead_nodraw    318ns ->  276ns    0.87x

It certainly cannot hurt performance.

BUG=chromium:470553

TBR=reed@google.com
No public API changes.

Review URL: https://codereview.chromium.org/1098183003
3 files changed