Battery Status API: add UMA logging for Mac.

Battery Status API: add UMA logging for Mac.

Adds UMA logging to track how many batteries are reported by
the system at the start of Battery Status API.

BUG=

Committed: https://crrev.com/07d1ece2458e41beb8b95e1c0613fdf1da0864bf
Cr-Commit-Position: refs/heads/master@{#295042}

[timvolodine, 2014-09-04 14:36:45]

timvolodine@chromium.org changed reviewers:
+ mvanouwerkerk@chromium.org

[Michael van Ouwerkerk, 2014-09-04 14:46:15]

lgtm

[timvolodine, 2014-09-04 17:33:20]

timvolodine@chromium.org changed reviewers:
+ isherman@chromium.org

[timvolodine, 2014-09-04 17:33:21]

+ isherman@ for histograms.xml

[Ilya Sherman, 2014-09-04 23:02:48]

https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
tools/metrics/histograms/histograms.xml:1955: +</histogram>
Why is this a separate histogram by platform, rather than having each platform
emit to a shared histogram?

[timvolodine, 2014-09-05 11:44:43]

https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
tools/metrics/histograms/histograms.xml:1955: +</histogram>
On 2014/09/04 23:02:48, Ilya Sherman wrote:
> Why is this a separate histogram by platform, rather than having each platform
> emit to a shared histogram?

the reason is that different platforms provide different information, e.g. afaik
on android there is no way to know how many batteries are in the system. Also we
want a breakdown by platform to see where we need to provide multi-battery
support, e.g. there is a suspicion that for mac the number of devices with
multiple batteries will be zero (or close to zero).

[timvolodine, 2014-09-05 13:12:23]

On 2014/09/05 11:44:43, timvolodine wrote:
>
https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
> File tools/metrics/histograms/histograms.xml (right):
> 
>
https://codereview.chromium.org/545503002/diff/1/tools/metrics/histograms/his...
> tools/metrics/histograms/histograms.xml:1955: +</histogram>
> On 2014/09/04 23:02:48, Ilya Sherman wrote:
> > Why is this a separate histogram by platform, rather than having each
platform
> > emit to a shared histogram?
> 
> the reason is that different platforms provide different information, e.g.
afaik
> on android there is no way to know how many batteries are in the system. Also
we
> want a breakdown by platform to see where we need to provide multi-battery
> support, e.g. there is a suspicion that for mac the number of devices with
> multiple batteries will be zero (or close to zero).

Michael remarked that we can slice by platform in UMA, so to clarify a bit more:
on android and windows we would have "0" or "1+" histograms (don't know about
chromeOS yet).

[Ilya Sherman, 2014-09-09 22:17:27]

Ok.  I'd recommend using a single histogram for all platforms where it makes
sense; but if you really prefer to have separate histograms per platform, then
LGTM.

[timvolodine, 2014-09-10 14:48:37]

On 2014/09/09 22:17:27, Ilya Sherman wrote:
> Ok.  I'd recommend using a single histogram for all platforms where it makes
> sense; but if you really prefer to have separate histograms per platform, then
> LGTM.

thanks!
also fixed the code to update the histogram only on start (not on each battery
update)..

[timvolodine, 2014-09-16 09:41:32]

The CQ bit was checked by timvolodine@chromium.org

[commit-bot: I haz the power, 2014-09-16 09:42:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/545503002/20001

[commit-bot: I haz the power, 2014-09-16 10:51:08]

Committed patchset #2 (id:20001) as 1ce46fbda4f56a94b666a393c26a3b5c0c35d2ce

[commit-bot: I haz the power, 2014-09-16 10:52:41]

Patchset 2 (id:??) landed as
https://crrev.com/07d1ece2458e41beb8b95e1c0613fdf1da0864bf
Cr-Commit-Position: refs/heads/master@{#295042}