Reland: Add Stop() to AudioDecoder. (https://codereview.chromium.org/126793002/)

BUG=329379, 335181

Review URL: https://codereview.chromium.org/140823012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246529 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246529