Don't change the format of the audio before stopping the old audio source.

Otherwise the size of the fifo in the audio track is not setup properly, I guess this is the potential cause for bug 304908.

BUG=304908
R=tommi@chromium.org

Review URL: https://codereview.chromium.org/26838002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227939 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227939