The perf tests results are used by tools/perf-to-html.py to generate
html pages. Since the results are used as divisors to compute
percentages, when the one of them happens to be zero, the script
crashes.  This CL prevent the crash and uses the text "NaN" instead of
the percentage.

R=machenbach@chromium.org
CC=​ahaas@chromium.org,clemensh@chromium.org

Bug: chromium:761816
Change-Id: I482a85150c8323a7a837e6d589feee88279831cd
Reviewed-on: https://chromium-review.googlesource.com/649626
Commit-Queue: Enrico Bacis <enricobacis@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47806}