Support nullable buffer for AudioBufferSourceNode.

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 91322e20e5b0545980e31c72cd2375697cb56095..81526b1e9d2484e5ddf99de599a3b418881ade0d 100644
--- a/LayoutTests/webaudio/audiobuffersource-channels.html
+++ b/LayoutTests/webaudio/audiobuffersource-channels.html
@@ -29,7 +29,18 @@ function runTest() {
 
     // Make sure we can't set to something which isn't an AudioBuffer.
     shouldThrow("source.buffer = 57", '"TypeError: Failed to set the \'buffer\' property on \'AudioBufferSourceNode\': The provided value is not of type \'AudioBuffer\'."');
-    shouldThrow("source.buffer = null", '"TypeError: Failed to set the \'buffer\' property on \'AudioBufferSourceNode\': buffer cannot be null"');
+
+    // Check that mono buffer can be set.
+    try {
+        var nullableBuffer = null;
+        source.buffer = nullableBuffer;
+        if (source.buffer.numberOfChannels == 1)
+            testPassed("Nullable buffer can be set.");
+        else
+            testFailed("Nullable buffer must be mono.");
+    } catch(e) {
+        testFailed("Nullable buffer can not be set.");
+    }
 
     // Check that mono buffer can be set.
     try {