Repeat Chromium Tryserver page_set runs to reduce variance.

* Find the median of runs with the same page_name in csv_merger.py
* vm_run_telemery.sh will now read an env variable to determine how many times each page_set run should be repeated.

BUG=358628, skia:2396

Review URL: https://codereview.chromium.org/231433003
12 files changed