Thread through a flag to force SkPicture::playback() when recording subpictures.

This makes nanobench picture recording benchmarks somewhat useful again,
as opposed to all taking about 5us to run no matter the content.

ATTN Sheriff: this will probably trigger perf.skia.org alerts.

BUG=skia:

Review URL: https://codereview.chromium.org/1219873002
5 files changed