Lazy init for DefaultAudioDestinationNode and OfflineAudioDestinationNode

Lazy init for DefaultAudioDestinationNode and OfflineAudioDestinationNode
https://bugs.webkit.org/show_bug.cgi?id=76509

Patch by Raymond Liu <raymond.liu@intel.com> on 2012-02-16
Reviewed by Eric Seidel.

No new tests required.

* webaudio/AudioDestinationNode.h:
* webaudio/AudioNode.h:
(WebCore::AudioNode::sampleRate):
* webaudio/DefaultAudioDestinationNode.cpp:
(WebCore::DefaultAudioDestinationNode::DefaultAudioDestinationNode):
* webaudio/DefaultAudioDestinationNode.h:
* webaudio/OfflineAudioDestinationNode.cpp:
(WebCore::OfflineAudioDestinationNode::OfflineAudioDestinationNode):
* webaudio/OfflineAudioDestinationNode.h:
(WebCore::OfflineAudioDestinationNode::sampleRate):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108022 268f45cc-cd09-0410-ab3c-d52691b4dbfc