Since ChromeOS v48, some videos when played back from Google Drive fail at certain points in playback. To get it to start again, you have to reload the page and skip past the section causing the issue. This has been flagged with the Chromium developer team and is down for a fix in v51. However, as a temporary workaround, in chrome://flags disable "Hardware-accelerated video decode Mac, Windows, Chrome OS" and restart. The videos will then play fine. This setting has to be changed on a per user basis. Brief video of changing the flag: