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This change aims to demonstrate how we can re-implement the following
methods in a sane way.

 * TSFTextStore::CancelComposition()
 * TSFTextStore::ConfirmComposition()

In our initial implementation [1], those two methods were implemented
in a really hacky way. It was implemented by simulating a text change
event during a composition with an assumption that the current IME will
terminate the composition when it receives |OnTextChange|.
|ITfContextOwnerCompositionServices::TerminateComposition| should have
been used instead in this scenario.

[1]: e7888801

Bug: 948783
Change-Id: Ibfb270d9f8dc2459ca73824c7405f10111e12ad1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572240
Commit-Queue: Siye Liu <siliu@microsoft.com>
Reviewed-by: Yohei Yukawa <yukawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652873}