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
2 files changed
tree: 6803c466e3a653f4501e3c8abff93e3eb8041d8a
  1. third_party/