Auto merge of #22648 - collares:ChannelSplitterNode, r=Manishearth

Implement DOM APIs for ChannelSplitterNode

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Based on #21591. Fixes #21558. I tried to update the expected results for WPT using "./mach update-wpt"; let me know if I got something wrong.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix the second half of #21558

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- [x] There are web-platform-tests tests for these changes
- [ ] These changes do not require tests because ___

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components/script/dom/webidls/AudioNode.webidl

@@ -57,5 +57,6 @@ interface AudioNode : EventTarget {
   attribute unsigned long channelCount;
   [SetterThrows]
   attribute ChannelCountMode channelCountMode;
+  [SetterThrows]
   attribute ChannelInterpretation channelInterpretation;
 };

components/script/dom/webidls/BaseAudioContext.webidl

@@ -45,7 +45,7 @@ interface BaseAudioContext : EventTarget {
   [Throws] PannerNode createPanner();
   // StereoPannerNode createStereoPanner();
   // ConvolverNode createConvolver();
-  // ChannelSplitterNode createChannelSplitter(optional unsigned long numberOfOutputs = 6);
+  [Throws] ChannelSplitterNode createChannelSplitter(optional unsigned long numberOfOutputs = 6);
   [Throws] ChannelMergerNode createChannelMerger(optional unsigned long numberOfInputs = 6);
   // DynamicsCompressorNode createDynamicsCompressor();
   [Throws]  OscillatorNode createOscillator();

components/script/dom/webidls/ChannelSplitterNode.webidl

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://webaudio.github.io/web-audio-api/#channelsplitternode
+ */
+
+dictionary ChannelSplitterOptions : AudioNodeOptions {
+  unsigned long numberOfOutputs = 6;
+};
+
+[Exposed=Window,
+ Constructor (BaseAudioContext context, optional ChannelSplitterOptions options)]
+interface ChannelSplitterNode : AudioNode {
+};