HTMLMediaElement::beginScrubbing() unconditionally pauses playback; this means that clicking the time-bar of an unseekable video does a pause/play pair without moving currentTime (b/c of 4.8.10.9, step 7). Instead, interacting with the time-bar of unseekable media should be a true no-op and playback should be smooth. (found this while fixing http://crbug.com/60691)