Fixes crash due to NullAudioRenderer calling GetPlaybackRate() during initialization.
Fixes crash due to NullAudioRenderer calling GetPlaybackRate() during initialization.

This mostly affects test_shell as NullAudioRenderer is only used with chrome when passing --disable-audio.

BUG=13907
TEST=test should crash less

Review URL: http://codereview.chromium.org/172018

git-svn-id: http://src.chromium.org/svn/trunk/src@23507 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 979f9d411fdab3f0b88d844c292f1d4da2a2c70c
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