Simplify AudioRendererImpl by using AudioDevice.

      This helps us move closer to being able to do renderer-side mixing, for
improved performance.
See original CL for discussion: http://codereview.chromium.org/8477037/

BUG=none
TEST=audio_renderer_impl_unittest
(also verified that the media layout tests all pass, and did manual
testing with several audio files and YouTube videos)

Review URL: http://codereview.chromium.org/8785008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113821 0039d316-1c4b-4281-b951-d872f2087c98