We can improve the way we handle passing around and identifying readystatechange and progress events in the XMLHttpRequest classes to reduce technical debt.

1. Factor out the enums and string constants into the base XMLHttpRequest class.
2. Add a few helper methods to make it easier to compare, loop through, and get the string representation of an event type.
3. Use stronger typing on a couple of methods to ensure they only get the types of events they expect.

This will also make the final code a bit more readable and easier to alter in bugs like bug 1842970, as well as making it trivial to print actual event names rather than enum integers in our debug logs.