[autotest] Record suite runtime

Measure the total time taken to run a suite from the start to the end.
Record the data in metadata DB.

BUG=chromium:426091
TEST=./site_utils/run_suite_unittest.py

Change-Id: Ia5fc8997d6c10687bd484b0a9486bb17a04edba0
Reviewed-on: https://chromium-review.googlesource.com/225234
Reviewed-by: Mungyung Ryu <mkryu@google.com>
Commit-Queue: Mungyung Ryu <mkryu@google.com>
Tested-by: Mungyung Ryu <mkryu@google.com>
3 files changed