In order to support IA2 live regions with e10s (bug 1322532), text change events sent from the child process to the parent process use sync IPC.
That comes at a slight performance cost: the content process is blocked until the parent returns from sending and handling the event.
However, there is no reason to do this if the event is not for an accessible inside a live region; current clients don't need the sync behaviour outside of live regions.
Differential Revision: https://phabricator.services.mozilla.com/D15183
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extra : source : 415f827cb9ce211c48b0c6625e917582bc81fd3a
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