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

Reason for revert:
Causes blink layout test (media/encrypted-media/encrypted-media-needkey.html) to crash:

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&showExpectations=true&tests=media%2Fencrypted-media%2Fencrypted-media-needkey.html

Original issue's description:
> Add Stop() to AudioDecoder.
> 
> BUG=329379
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244994

TBR=dalecurtis@chromium.org,scherkus@chromium.org,xhwang@chromium.org,rileya@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=329379

Review URL: https://codereview.chromium.org/139513007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245083 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245083