Let the effective media volume be 0 if the media element is muted

The bug was with a muted media element slaved to an unmuted media
controller, in which case the media element's muted state was ignored.

Attaching a media controller to an already-playing media element also
wouldn't update the volume, so do that in bringElementUpToSpeed().

BUG=399623

Review URL: https://codereview.chromium.org/444173002

git-svn-id: svn://svn.chromium.org/blink/trunk@179738 bbb929c8-8fbe-4397-9dbb-9b2b20218538