Return 0 for confidence when there's only results for one "good" or one "bad" rev.

Return 0 for confidence when there's only one good or bad rev.

BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288982

[qyearsley, 2014-08-12 01:13:34]

Also in this CL:

Extract out a constant which represents the boundary between "low" and
"relatively high" confidence, and stop warning that "Confidence is less than
100%".

[tonyg, 2014-08-12 02:28:02]

lgtm

TBH, I'm on the fence about whether this should really be 0 or just some other
low number. But okay.

[qyearsley, 2014-08-12 04:45:45]

On 2014/08/12 02:28:02, tonyg wrote:
> lgtm
> 
> TBH, I'm on the fence about whether this should really be 0 or just some other
> low number. But okay.

Aye... using another low number would fit better with our intuition of what
"confidence" is supposed to represent... after all, it's definitely still
*possible* that the suspected CL is actually the culprit CL. Any other number
seems a little bit arbitrary though.

[qyearsley, 2014-08-12 04:56:50]

The CQ bit was checked by qyearsley@chromium.org

[commit-bot: I haz the power, 2014-08-12 04:58:07]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/qyearsley@chromium.org/463743002/20001

[commit-bot: I haz the power, 2014-08-12 15:30:01]

Change committed as 288982