Update retrieve_story_timing script to round the time duration and update the setup instruction

NOTRY=true
TBR=eyaich@chromium.org

Bug: 863768
Cq-Include-Trybots: master.tryserver.chromium.perf:obbs_fyi
Change-Id: I942eb00c9a4c5b0c4bb5b370f845b3d20cd97368
Reviewed-on: https://chromium-review.googlesource.com/1138397
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#575264}