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They used to be separate because Base acted as a base class for all
platforms, but we've since removed support for the other platforms [1].

Also remove some non-ChromeOS code from the class since it's dead code.

Follow up CLs will combine InputMethodEngineBase with InputMethodEngine.

[1] https://crrev.com/c/2198861

Bug: 1083803
Change-Id: Ia49ac04c161df37a42ed5e9bd5ac41b052b453b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212194Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772110}