Bug 827708 - [music] the seeking position is not always the same as the users tapping position

https://bugzilla.mozilla.org/show_bug.cgi?id=827708

Root Cause:
There are two elements called #player-seek-bar and #player-seek-bar-progress will trigger seek event. It let users sometimes touch #player-seek-bar and sometimes touch #player-seek-bar-progress.

And the width size of these two elements are different. So the seek function will return wrong result, and this bug will happen.

Solution:
Disable event trigger function of #player-seek-bar-progress element.