Bug
1753173 - part 2: Make `IMContextWrapper` set IME focus at receiving `NOTIFY_IME_OF_FOCUS` r=m_kato
Currently, it sets "input-purpose" and "input-hints" and notify IME of focus
at updating `InputContext`. This occurs before receiving `NOTIFY_IME_OF_FOCUS`.
Therefore, at the moment, `IMContextWrapper` cannot access content cache, but
IME may query it. E.g., IME may start composition immediately
(ibus-pinyin 1.5.0-6 does it).
For avoiding the trouble, `IMContextWrapper` should notify IME of focus when
and only when it receives `NOTIFY_IME_OF_FOCUS`.
Differential Revision: https://phabricator.services.mozilla.com/
D137610