graphAnalyzer: refine stat.hasVideo

[esaruoho]

originally posted by @gmarinov

since iOS10, silent videos are allowed to autoplay

graphAnalyser.js:

if (videoPlugins.indexOf(id) > -1) {
  stat.hasAudio = true
  stat.hasVideo = true
}

... marks all experiences using video with .hasAudio=true regardless of whether the video actually has sound. It would be beneficial to check the video asset(s) in question.

more information: https://webkit.org/blog/6784/new-video-policies-for-ios/