bindings: Don't wrap nullable callback functions in Nullable<T>

The code generator wrongly uses `Nullable<Callback*>` when nullable
callback functions are used like below:

callback C = void();
interface I { void f(C? callback); }

Since C++ impl of callback functions are garbage collected types,
we can just use `Callback*`.

BUG=675859

Review-Url: https://codereview.chromium.org/2588303002
Cr-Commit-Position: refs/heads/master@{#439816}

third_party/WebKit/Source/bindings/scripts/v8_types.py

@@ -965,10 +965,14 @@ def cpp_type_has_null_value(idl_type):
     #   a null pointer.
     # - Union types, as thier container classes can represent null value.
     # - 'Object' and 'any' type. We use ScriptValue for object type.
-    return (idl_type.is_string_type or idl_type.is_interface_type or
-            idl_type.is_enum or idl_type.is_union_type
-            or idl_type.base_type == 'object' or idl_type.base_type == 'any'
-            or idl_type.is_custom_callback_function or idl_type.is_callback_interface)
+    return (idl_type.is_string_type
+            or idl_type.is_enum
+            or idl_type.is_interface_type
+            or idl_type.is_callback_interface
+            or idl_type.is_callback_function
+            or idl_type.is_custom_callback_function
+            or idl_type.is_union_type
+            or idl_type.base_type == 'object' or idl_type.base_type == 'any')
 
 IdlTypeBase.cpp_type_has_null_value = property(cpp_type_has_null_value)
 

third_party/WebKit/Source/bindings/tests/results/core/V8TestCallbackFunctions.cpp

@@ -249,7 +249,7 @@ static void voidMethodNullableCallbackFunctionInArgMethod(const v8::FunctionCall
     return;
   }
 
-  Nullable<VoidCallbackFunction*> voidCallbackFunctionArg;
+  VoidCallbackFunction* voidCallbackFunctionArg;
   if (!info[0]->IsFunction() && !info[0]->IsNull()) {
     V8ThrowException::throwTypeError(info.GetIsolate(), ExceptionMessages::failedToExecute("voidMethodNullableCallbackFunctionInArg", "TestCallbackFunctions", "The callback provided as parameter 1 is not a function."));