For sending NOTIFY_IME_OF_COMPOSITION_EVENT_HANDLED after the other change notifications which was caused by the user input, we need to use IMEContentObserver::IMENotificationSender because it sends the notifications when it's safe to do it.
This patch makes TextComposition use IMEContentObserver to send the notification. However, if there is no active IMEContentObserver, e.g., composition events are fired on unfocused window, TextComposition sends it by itself (same as current implementation).
If IMEContentObserver stops observing when it has pending NOTIFY_IME_OF_COMPOSITION_EVENT_HANDLED, it cannot send the notification (i.e., it is discarded completely in such case). However, in such case, IMEContentObserver sends NOTIFY_IME_OF_BLUR. So, anyway, native IME handler should treat the blur notification as it including NOTIFY_IME_OF_COMPOSITION_EVENT_HANDLED.
On the other hand, we're buggy if composition events are fired in non-active window. Even in such case, IMEContentObserver should be created for active editor in each document and it notifies IME of the changes. But this is out of the scope of this bug.
MozReview-Commit-ID: 7Q0ZsJTh4hX
--HG--
extra : rebase_source : 6417f991fa8c0fbe3f25b27bacf4257e5485aecc
54e57600e1