Measure usage of MediaStream.prototype.active

The active/inactive events have been removed from the spec, but the
active attribute has not, so removing just the events wouldn't reduce
complexity by much. Gecko doesn't have the attribute either, so measure
its usage before deciding on the next steps.

Related spec issue:
https://github.com/w3c/mediacapture-main/issues/370

R=guidou@chromium.org,isherman@chromium.org

Review-Url: https://codereview.chromium.org/2128593003
Cr-Commit-Position: refs/heads/master@{#404352}
3 files changed