Notify IME on surrounding text update from exo.

Currently, it is not, so that IME does not have any way to
know the surrounding text update, so that it sometimes taking a
look at older surrounding text and makes misbehaviors.

BUG=1227590
TEST=Ran on DUT. Ran exo_unittests.

Change-Id: If7eae9636666218b171a68b055479a9e3a550a01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3276215
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#942399}