This is used to count the potiential number of the blocked autoplay media element without audio track
even if user was enable autoplay.
It might happen on three cases,
1. play -> loadedmetadata
2. loadedmetadata -> play
3. loadedmetadata -> has 'autoplay' keyword
In first case we need to check whether the play invocation has been called, and check other other cases
before the media starts playing.
In addition, the scalar name isn't consist with other names is because of the 40 maximum limitation of
the ping name.
MozReview-Commit-ID: 6Qm6TD4ME8I
--HG--
extra : rebase_source : d6c0dab7a0a2deed0025a0d039ead1f6ad218300
07c3b57fa1