ChannelMergerNode may need check for deferred updating of output channels
https://bugs.webkit.org/show_bug.cgi?id=108863

There can in rare cases be a slight delay before the output bus is updated
to the new number of channels because of tryLocks() in the context's
updating system. So need to check the channel number before processing.

Reviewed by Chris Rogers.

* Modules/webaudio/ChannelMergerNode.cpp:
(WebCore::ChannelMergerNode::ChannelMergerNode):
(WebCore::ChannelMergerNode::process):
(WebCore::ChannelMergerNode::checkNumberOfChannelsForInput):
* Modules/webaudio/ChannelMergerNode.h:
(ChannelMergerNode):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143565 268f45cc-cd09-0410-ab3c-d52691b4dbfc