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

Review URL: https://codereview.chromium.org/1371933005

Cr-Commit-Position: refs/heads/master@{#351823}
4 files changed