CharacterBoundsUpdateEventInit needs to have either required properties or they need to have default values

[smaug----]

... since otherwise the value of the attributes in the relevant Event isn't defined.

Chrome doesn't follow the current spec but seems to default to 0
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/editing/ime/character_bounds_update_event.cc;l=13
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/editing/ime/character_bounds_update_event.h;l=38