Intent to ship: dispatching "keydown" and "keyup" events during IME composition

[Masayuki Nakano]

Summary: We'll start to dispatch "keydown" and "keyup" events during IME
composition.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=354358

Link to standard:
https://w3c.github.io/uievents/#events-composition-key-events

Platform coverage: All

Estimated or target release: 65

DevTools bug: N/A

Do other browser engines implement this?

Yes, the other browsers use this behavior traditionally, and UI Events
declared the behavior as "standard".

web-platform-tests: N/A due to requiring user input with IME, but we
have mochitests with synthesized events.

This new behavior was enabled in Nightly and Early-Beta since 61. This
behavior is mainly expected by Korean web developers but we've not
implemented this for a long time.

For reducing the testing and developing cost of each web app vendor,
we've waited to ship keypress event changes which will be mentioned in
following posts.

Enabling patch has been landed from:
https://bugzilla.mozilla.org/show_bug.cgi?id=1496288
because we need to manage those changes as a set.