Accumulate log output instead of overwriting previous runs.

Accumulate log output instead of overwriting previous runs.

Previously, if runs failed their log output could get overwritten by later runs, making it harder to figure out what exactly went wrong, see crbug.com/511279. In that instance, the first 2 runs crash, and the final run times out with no output. Instead, we can accumulate the output of all the runs and output it.

Example output: http://simonhatch0.mon.corp.google.com:8013/builders/Android%20Nexus7v2%20Perf/builds/640
BUG=511279

Review URL: https://codereview.chromium.org/1264553003

Cr-Commit-Position: refs/heads/master@{#340961}