Sheriff-o-matic: Support LEAK failure type

Before this patch, sheriff-o-matic didn't show any information for LEAK
failures from "WebKit Linux Leak" builder. This patch changes it to
display object leak log.

BUG=404030
win_blink_rel seems broken atm. Passing on all other trybots and the trybots don't seem to trigger the test anyway.
NOTRY=true

Review URL: https://codereview.chromium.org/481833003

git-svn-id: svn://svn.chromium.org/blink/trunk@180509 bbb929c8-8fbe-4397-9dbb-9b2b20218538