nanobench: Add per-run time reporting.

This patch adds per-benchmark-iteration times to our JSON output. Given that we
already collect these statistics, giving them to the user would be nice.

No unit-test provided, since `rgrep -i json tests` yielded nothing. Happy to add
one if someone wants.

BUG=None.
TEST=nanobench now writes per-run timinings with the output JSON.

Change-Id: I910f1d97fd3e0ee69fc8e78e011e67b9c866f18d
Reviewed-on: https://skia-review.googlesource.com/5617
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Ravi Mistry <rmistry@google.com>
2 files changed