Intent to remove UIEvent.isChar

[Masayuki Nakano]

UIEvent.isChar was (probably) designed for that web apps can distinguish
the key combination inputs character(s).

However, this is initialized only on macOS (always false on the other
platforms) and other browsers don't support this.

Unfortunately, Add-on SDK checked this value. Currently, you see garbage
code of referring this here:
https://dxr.mozilla.org/mozilla-central/rev/f9362554866b327700c7f9b18050d7b7eb3d2b23/addon-sdk/source/lib/sdk/keyboard/hotkeys.js#72
However, nobody refers this variable now.

Some existing add-ons have old code of this. They refer the value but
fortunately, the behavior after removing this attribute won't be changed
because when it's false, they refer keyCode value but the value is
always false even with current build except on macOS.

So, I'd like to remove this non-standard attribute from Gecko.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1347073

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