Currently, the overlapping results of individual benchmark tests are overwritten.

Example:
  tools/perf-compare.py Result,ss.json,kr.json Master,kr-m.json,ss-m.json -o compare.html
or
  tools/perf-compare.py Result,ss.json Result,kr.json Master,kr-m.json Master,ss-m.json -o compare.html

BUG=v8:6144
NOTRY=true

Change-Id: Ia340e8c01c46da17b1f4eee6c6bb8e5e9bb12c3e
Reviewed-on: https://chromium-review.googlesource.com/459537
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44090}