Take two at fixing duplicate alert failures.

r183289 caused a new overwriting alerts bug because it didn't replicate
the way builder_alerts handled null failure reasons. Using step::reason
won't actually be unique if the reason is null since builder_alerts
doesn't merge failures with null reasons.

R=leviw@chromium.org,jyasskin@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/638333002

git-svn-id: svn://svn.chromium.org/blink/trunk@183831 bbb929c8-8fbe-4397-9dbb-9b2b20218538