Quality-of-life improvements for attempts to autoplay

[roscoemcinerney]

In response to https://developers.google.com/web/updates/2017/09/autoplay-policy-changes, some changes to HTMLVideo:

• Use the promise returned by HTMLMediaElement.play() in modern browsers to better expose success or failure when calling vastplayer.startAd()
• Set attribute "muted" on HTML5 video element when volume is set to 0, as modern browsers allow autoplay on explicitly muted videos even with low or zero Media Engagement Index - but not on videos with volume == 0.