GN: nanobench

Add nanobench, and while we're at it monobench to show off how cool
source_sets are... the bench files are only built once then linked
into both binaries.  With GYP we build them twice. :/  Same deal
for GMs between nanobench and DM... build once, link twice.

nanobench uses SkImageEncoder to encode its .pngs, which requires
we link in the image encoders, which requires we get them all in.
That's the bulk of this.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2193513002

Review-Url: https://codereview.chromium.org/2193513002
2 files changed