Fix the issue with adding/removing event listeners for Device Motion/Orientation.

Make sure to only stop updating Device Motion/Orientation when
there are no event listeners. Currently Device Motion/Orientation
stops updating upon first event listener removal which is
problematic in case multiple event listeners are registered.

BUG=364491
TEST=Layout test and also http://jsbin.com/motion.

Review URL: https://codereview.chromium.org/240993003

git-svn-id: svn://svn.chromium.org/blink/trunk@172165 bbb929c8-8fbe-4397-9dbb-9b2b20218538