Destroy existing AudioManager instances before creating new ones.

BUG=327013
TEST=no more crashes

Review URL: https://codereview.chromium.org/93793008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239651 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239651