Clarify how CompositionEvents should be initialized

[ltenmoz]

Currently the language around firing compositionstart/end events is just:

Fire an event named compositionstart/compositionend at editContext using CompositionEvent.

But this doesn't make it clear how the view and data attributes should be initialized. There is the table in the UI Events spec, but that seems to only apply to non-EditContext composition events on elements. Probably the EditContext spec should be explicit about this.

[smaug----]

@dandclark

[ltenmoz]

In particular, Chrome sets data to the current composition text for compositionstart, which is different from its behaviour for contenteditable/input/textarea (where data is always an empty string for compositionstart).