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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tests/wpt/mozilla/meta/MANIFEST.json

@@ -27080,7 +27080,7 @@
    "testharness"
   ], 
   "mozilla/interfaces.html": [
-   "8153d67c8cb4e554b05ac884e9898f50508349be", 
+   "95ab0109c82c8e90a3e53a3579b9337e2091e26c",
    "testharness"
   ], 
   "mozilla/interfaces.js": [

tests/wpt/mozilla/tests/mozilla/interfaces.html

@@ -30,6 +30,7 @@ test_interfaces([
   "CanvasRenderingContext2D",
   "CanvasPattern",
   "ChannelMergerNode",
+  "ChannelSplitterNode",
   "CharacterData",
   "CloseEvent",
   "CSS",