Only create AudioBus with required number of channels for AudioNodeOutput

Only create AudioBus with required number of channels for AudioNodeOutput
https://bugs.webkit.org/show_bug.cgi?id=76417

Patch by Raymond Liu <raymond.liu@intel.com> on 2012-01-18
Reviewed by Kenneth Russell.

No new tests required.

* webaudio/AudioNodeOutput.cpp:
(WebCore::AudioNodeOutput::AudioNodeOutput):
(WebCore::AudioNodeOutput::setNumberOfChannels):
(WebCore::AudioNodeOutput::updateInternalBus):
(WebCore::AudioNodeOutput::updateNumberOfChannels):
(WebCore::AudioNodeOutput::pull):
* webaudio/AudioNodeOutput.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105373 268f45cc-cd09-0410-ab3c-d52691b4dbfc