Fixes bug where NullAudioRenderer would join a previously joined thread.
Fixes bug where NullAudioRenderer would join a previously joined thread.

TBR=hclam

Review URL: http://codereview.chromium.org/100135

git-svn-id: http://src.chromium.org/svn/trunk/src@14809 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 4348e93f808967fd5bcb8ce03f48e6b27653cb07
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser