Remove lock from OnMoreDataConverter.

AudioOutputStream::Stop() is guaranteed to wait for outstanding
callbacks.  Since we call Stop() prior to modifying the callback
there's no reason to have an additional lock.

Found while auditing potential areas for deadlock conditions.

BUG=none
TEST=Unit tests pass, no crazy crashes.
R=miu@chromium.org

Review URL: https://codereview.chromium.org/62953017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234589 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234589