Make perf test runner more robust.

Convert all results to floats and log errors on failing
conversions. Otherwise these results bubble up into
buildbot.

TBR=tandrii@chromium.org
BUG=chromium:374740
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/811483008

Cr-Commit-Position: refs/heads/master@{#25913}