Do not copy and read data when the volume of the local renderer is 0.

This saves one interleave and two memcpy, which might be helpful for Clank.

TEST=verify http://webrtc.googlecode.com/svn/trunk/samples/js/demos/html/pc1.html works
R=henrika@chromium.org, jar@chromium.org

Review URL: https://codereview.chromium.org/24262004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224990 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224990