Use correct unsigned long attributes in AudioBuffer

The length attribute was defined as a long instead of unsigned long.
We also note that channelIndex for getChannelData(),
copyFromChannel(), and copyToChannel() was declared to be a long
instead of unsigned long.  Fix those too.

Bug: 1059305
Change-Id: Ia57fc606e8943de56bb2943b00e19b8beaeb9642
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095212Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748698}

third_party/blink/renderer/modules/webaudio/audio_buffer.idl

@@ -31,13 +31,20 @@
     Exposed=Window
 ] interface AudioBuffer {
     [RaisesException, Measure] constructor(AudioBufferOptions options);
-    readonly attribute long length; // in sample-frames
+    readonly attribute unsigned long length; // in sample-frames
     readonly attribute double duration; // in seconds
     readonly attribute float sampleRate; // in sample-frames per second
 
     // Channel access
     readonly attribute unsigned long numberOfChannels;
-    [HighEntropy, Measure, RaisesException] Float32Array getChannelData(unsigned long channelIndex);
-    [HighEntropy, Measure, RaisesException] void copyFromChannel(Float32Array destination, long channelNumber, optional unsigned long bufferOffset = 0);
-    [RaisesException] void copyToChannel(Float32Array source, long channelNumber, optional unsigned long bufferOffset = 0);
+    [HighEntropy, Measure, RaisesException] Float32Array getChannelData(
+        unsigned long channelIndex);
+    [HighEntropy, Measure, RaisesException] void copyFromChannel(
+        Float32Array destination,
+        unsigned long channelNumber,
+        optional unsigned long bufferOffset = 0);
+    [RaisesException] void copyToChannel(
+        Float32Array source,
+        unsigned long channelNumber,
+        optional unsigned long bufferOffset = 0);
 };