IDL compiler: [CustomLegacyCall] interfaces, cleanup header conditional spacing

Source/bindings/templates/callback_interface.h

Index: Source/bindings/templates/callback_interface.h
diff --git a/Source/bindings/templates/callback_interface.h b/Source/bindings/templates/callback_interface.h
index d0da29bea5f8c6207b4b084bad9dbf515dffa30b..eaf7529220bcdc3b26021c7685d497825967c196 100644
--- a/Source/bindings/templates/callback_interface.h
+++ b/Source/bindings/templates/callback_interface.h
@@ -35,9 +35,7 @@
 #ifndef {{v8_class_name}}_h
 #define {{v8_class_name}}_h
 
-{% if conditional_string %}
-#if {{conditional_string}}
-{% endif %}
+{% filter conditional(conditional_string) %}
 {% for filename in header_includes %}
 #include "{{filename}}"
 {% endfor %}
@@ -71,8 +69,5 @@ private:
 };
 
 }
-
-{% if conditional_string %}
-#endif // {{conditional_string}}
-{% endif %}
+{% endfilter %}
 #endif // {{v8_class_name}}_h