Add pre_log option to nanobench; make this option default true in nanobench_flags.py and dm_flags.py.

BUG=skia:4826
BUG=skia:4827
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1612483002

Review URL: https://codereview.chromium.org/1612483002
8 files changed