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This allows chrome://settings/inputMethods to update its contents
when a new Chrome OS or Android IME is added or removed.

BUG=867795,845079
TEST=try

Change-Id: Ibae94e01c14422ea5d2766b737397050aae9a299
Reviewed-on: https://chromium-review.googlesource.com/1195441Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588393}