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Previously, only BS/DEL set this guard but there are cases with keyboards
that send single characters immediately followed by a composition (e.g.
swiping words on stock Google keyboard) and we want to make sure that
there is no race condition where the actions taken in response to the
KeyPress don't cancel the following composition.

BUG=408678

Review URL: https://codereview.chromium.org/518123002

Cr-Commit-Position: refs/heads/master@{#293562}