bindings: Don't throw a TypeError when 'null' is passed to nullable callback function

third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html

Index: third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html
diff --git a/third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html b/third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html
index 4fb98497323658d36cdc2683ae515f35fcb0c6da..70e928c4ff075c0cfac6c2c21d07d3221c92cbcb 100644
--- a/third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html
+++ b/third_party/WebKit/LayoutTests/fast/dom/idl-callback-function-unittest.html
@@ -54,4 +54,8 @@ test(function() {
         callbackFunctionTest.testCallback(1, 'hello', 'world');
     });
 }, 'Passing non-callable values should throw a TypeError');
+
+test(function() {
+  assert_equals(callbackFunctionTest.testNullableCallback(null, 'hello', 'world'), 'Empty callback');
+}, 'Passing null to nullable callback argument');
 </script>