on previous versions of Android, we were be able to start a video by calling play() method on video js element or by simulating a click event on it (only for some specifics
Please try calling WebView.getSettings().setMediaPlaybackRequiresUserGesture(false) for your WebView.
WebView.getSettings().setMediaPlaybackRequiresUserGesture(false)
(see http://developer.android.com/reference/android/webkit/WebSettings.html#setMediaPlaybackRequiresUserGesture(boolean) )