skpbench: use accumulative result as the default

Uses accum instead of median for the first column in skpbench.cpp, and
the default result for parseskpbench.py. The accum is empirically
proving to be more repeatable than the median.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2378383002

Review-Url: https://codereview.chromium.org/2378383002
4 files changed