MediaStream Recorder: add usage counter to WebRTC.webkitApiCount UMA

MediaStream Recorder: add usage counter to WebRTC.webkitApiCount UMA

MediaStream Recorder is a new API and soon will
be enabled as WebKit experimental. This CL adds
a UMA usage counter as another WebRtc JS call
counter.

BUG=262211
Committed: https://crrev.com/2f4aeee97410f1d38ec1ca0927a7a71404620bdb
Cr-Commit-Position: refs/heads/master@{#351823}

[mcasas, 2015-09-30 22:18:12]

Patchset #1 (id:1) has been deleted

[mcasas, 2015-09-30 23:32:40]

mcasas@chromium.org changed reviewers:
+ asvitkine@chromium.org, perkj@chromium.org

[mcasas, 2015-09-30 23:32:40]

asvitkine@ histograms.xml PTAL

perkj@ I'm piggybacking on your UMA. PTAL :)

[perkj_chrome, 2015-10-01 10:57:07]

lgtm

[Alexei Svitkine (slow), 2015-10-01 16:23:31]

lgtm

[mcasas, 2015-10-01 16:35:49]

The CQ bit was checked by mcasas@chromium.org

[commit-bot: I haz the power, 2015-10-01 16:36:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371933005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371933005/20001

[commit-bot: I haz the power, 2015-10-01 16:42:18]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-10-01 16:43:14]

Patchset 1 (id:??) landed as
https://crrev.com/2f4aeee97410f1d38ec1ca0927a7a71404620bdb
Cr-Commit-Position: refs/heads/master@{#351823}