Auto merge of #21712 - Manishearth:analysernode, r=ferjm

Implement AnalyserNode

<s>Needs https://github.com/servo/media/pull/127 to land (and a dependency
update)</s>

r? @ferjm

realtimeanalyser-fft-scaling.html, the test that actually checks for some level of FFT correctness, sadly doesn't work since we don't process nodes not connected to the destination.

However I locally fixed the test to work differently and it passed. We'll fix the processing model eventually.

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tests/wpt/web-platform-tests/webaudio/the-audio-api/the-analysernode-interface/ctor-analyser.html

@@ -32,7 +32,7 @@
           prefix: prefix,
           numberOfInputs: 1,
           numberOfOutputs: 1,
-          channelCount: 1,
+          channelCount: 2,
           channelCountMode: 'max',
           channelInterpretation: 'speakers'
         });

tests/wpt/web-platform-tests/webaudio/the-audio-api/the-analysernode-interface/test-analysernode.html

@@ -27,8 +27,8 @@
 
       assert_equals(
         analyser.channelCount,
-        1,
-        "analyser node has 1 input channels by default"
+        2,
+        "analyser node has 2 input channels by default"
       );
       assert_equals(
         analyser.channelCountMode,
@@ -131,8 +131,8 @@
       var analyser = new AnalyserNode(context);
       assert_equals(
         analyser.channelCount,
-        1,
-        "analyser node has 1 input channels by default"
+        2,
+        "analyser node has 2 input channels by default"
       );
       assert_equals(
         analyser.channelCountMode,