Mac: Fix Japanese IME regression
Mac: Fix Japanese IME regression

This is a partial revert of https://codereview.chromium.org/1240943002.
Setting handling_input_event_ to true in OnImeSetComposition caused this
regression. This was needed to fix Korean IME on 10.11 (crbug.com/500376),
but when I tested with the latest 10.11 beta 15A244d, Korean IMA works just
fine without this change. Looks like Apple fixed something in the latest beta.

BUG=516882,500376

Review URL: https://codereview.chromium.org/1291253002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343347}

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