Add use counters for VTTCue use/rendering and its attributes

Having some data about how often WebVTT is used/rendered and which cue
attributes are mostly commonly used could prove valuable in spec
discussions, and is if nothing else interesting to know.

BUG=none

Review URL: https://codereview.chromium.org/263213002

git-svn-id: svn://svn.chromium.org/blink/trunk@173330 bbb929c8-8fbe-4397-9dbb-9b2b20218538