Perf Runner: Properly merge histograms. The prior method was emitting 5 histograms per test. The new script method merges the 5 histogram samples together and recomputes the running statistics. Should improve reporting on the dashboard. Bug: angleproject:6090 Change-Id: I58a5e020f3a3257a8545df2785bc3eaa811ee38e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3096550 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>