Swarming tasks: print start and end time

This will help determine how long each component of the task takes.

NOTRY=true
BUG=skia:4763
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1769583002

Review URL: https://codereview.chromium.org/1769583002
4 files changed