Bug 298656

Currently, the timestamp event parameter is mapped to "number", which results in a C++ double type, usually filled from something like `WallTime::now().secondsSinceEpoch().milliseconds()` which might still contain a fractional part. Meanwhile, both Selenium and WPT tests expect them to be a plain int in Python, which matches the spec definition of this field being a "js-uint". We could work around it by just adding some `std::trunc` call before passing the values to the generated code. But maybe we could add an annotation like `"integral": true` to the json file, so the generated code ensures this constraint.