Take two at fixing duplicate alert failures.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183831

[ojan, 2014-10-10 02:50:36]

ping

[ojan, 2014-10-16 04:14:10]

REVIEW MAH PATCHZ NOW UR ELSE

[leviw_travelin_and_unemployed, 2014-10-16 17:33:41]

The CQ bit was checked by leviw@chromium.org

[leviw_travelin_and_unemployed, 2014-10-16 17:33:42]

Okay dokay LGTM.

 _      _______ _______ _______ 
(_)    (_______|_______|_______)
 _      _   ___    _    _  _  _ 
| |    | | (_  |  | |  | ||_|| |
| |____| |___) |  | |  | |   | |
|_______)_____/   |_|  |_|   |_|

[commit-bot: I haz the power, 2014-10-16 17:34:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/638333002/1

[commit-bot: I haz the power, 2014-10-16 17:35:03]

Committed patchset #1 (id:1) as 183831