Intent to unship: DOM "text" event

[Masayuki Nakano]

Summary: We'll stop dispatching DOM "text" event which is Mozilla
specific DOM event and really not useful.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1288640

Link to standard: N/A

It's mentioned by MDN only here:
https://developer.mozilla.org/en-US/docs/Mozilla/IME_handling_guide#eCompositionChange

DevTools bug: N/A

Do other browser engines implement this? No.

web-platform-tests: N/A

This was implemented to notify our editor of composition string data and
IME selection ranges with DOM event and the information is available
only from C++ event listeners. So, this should not have been exposed to
the web since first implementation.

Additionally, we might need to change "compositionupdate" event
dispatcher if Input Events Level 2 spec would keep current definition.
Therefore, if we stop using DOM event to handle IME composition in our
editor, we can make maintain around it easier.

Currently, we should stop dispatching "text" events only in the default
group of web content. I.e., we should hide the event only from web apps.
If this won't cause any problem with the web, we should get rid of the
event completely later.

Note that this event is listened by only 0.0003% inner-windows for now
(currently, we collect the data only from Nightly testers).

--
Masayuki Nakano <masa...@d-toybox.com>
Software Engineer, Mozilla