nanobench: lazily decode bitmaps from SKPs

This makes it considerably cheaper to run SKP recording benchmarks, without
affecting their measurements and without really affecting SKP playback
benchmarks at all.

On my machine, running out/Release/nanobench --match skp --config nondrendering
drops in run time from 6.7s to 2.5s, and the peak RAM usage drops from 129M to 50M.

I'm strongly considering making this lazy decoding the default.

BUG=skia:
R=robertphillips@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/572933006
3 files changed