Get rid of custom bindings for AudioBufferSourceNode.buffer setter

LayoutTests/webaudio/audiobuffersource-channels.html

Index: LayoutTests/webaudio/audiobuffersource-channels.html
diff --git a/LayoutTests/webaudio/audiobuffersource-channels.html b/LayoutTests/webaudio/audiobuffersource-channels.html
index f24ed60322c216b979ece24c0839d62b290bfee7..e3b10ab8bd1fd28c173ad7d92cb43cbb4296c3db 100644
--- a/LayoutTests/webaudio/audiobuffersource-channels.html
+++ b/LayoutTests/webaudio/audiobuffersource-channels.html
@@ -28,7 +28,8 @@ function runTest() {
     source = context.createBufferSource();
 
     // Make sure we can't set to something which isn't an AudioBuffer.
-    shouldThrow("source.buffer = 57");
+    shouldThrow("source.buffer = 57", "'TypeError: Type error'");
+    shouldThrow("source.buffer = null", "'TypeError: buffer cannot be null'");
 
     // Check that mono buffer can be set.
     try {