[test-runner] Pass slowest test durations to buildbot.

TBR=jkummerow@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1073773002

Cr-Commit-Position: refs/branch-heads/4.3@{#5}
Cr-Branched-From: f5c0a23a-refs/heads/4.3.61@{#1}
Cr-Branched-From: 0a7d4f49-refs/heads/master@{#27508}