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

TBR=jkummerow@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1078693002

Cr-Commit-Position: refs/branch-heads/4.2@{#17}
Cr-Branched-From: 3dfd929e-refs/heads/4.2.77@{#2}
Cr-Branched-From: e0110920-refs/heads/master@{#26757}