Limit the number of output channels for a ChannelMergerNode.

Add a new test for this.

See the discussion in https://github.com/WebAudio/web-audio-api/issues/312. There appears to be general agreement that the number of outputs should be limited to 32 (or the implementation maximum) and any extras are ignored.

BUG=345174

Review URL: https://codereview.chromium.org/226663005

git-svn-id: svn://svn.chromium.org/blink/trunk@171678 bbb929c8-8fbe-4397-9dbb-9b2b20218538