Collect standard deviation for timing metrics

Authored by Peter on Mon, Apr 16, 8:54 AM.

Description

Collect standard deviation for timing metrics

Collecting sdev makes it easier for us to know about problematic URLs
and we can make sure everything works as we hope when we move the
WebPageTest instance to Docker.

Bug: T191615
Change-Id: Iec14f2aefaa3cdd43bcd63e4566915238a889295